That looks a lot like melted aluminum.
I'd use a solution of sodium hydroxide to dissolve it; probably won't even hurt the oven paint.
Easy
Off Heavy Duty Oven Cleaner is NaOH based, so that'll probably work for you, and is easier to find than straight lye nowadays.
Just follow the directions.
If the aluminum is thick, you may have to do things twice.
Other brands of oven cleaner will likely work as well as long as they contain sodium hydroxide (lye).
It will help A LOT for you people who need aluminum foil off your oven!
cooking.stackexchange.com/users/5455/Wayfaring Stranger
There was an article about How to Remove Melted Aluminum Foil from the Oven , but following several disappointing results by the participants, a user suggested a solution that was met with resounding success by those who tried it.
The author suggested an application of naval jelly (from the paint department at your local hardware store) as the solution that actually worked.
Naval jelly is pink goop containing phosphoric acid that is used to remove rust from iron or steel, leaving a clean surface.
Essentially, you spread aluminum jelly over the aluminum, smear it around every few hours, leave it overnight, wipe clean and repeat process until the residue is gone.
It might take a few days, but it is a gentle process with no fumes.
The solution was was verified enthusiastically by several users in that conversation.
You can read more about the process and reviews starting here:
BK says: Back in November, I searched for a way to fix our new oven … Bonus solution:
The original article also claims that the entire bottom plate of your oven can be replaced usually for less than US$30.00.
Contact your manufacturer for a replacement part and guidance on installing it.
cooking.stackexchange.com/users/8/Robert Cartaino
Trying to burn it off with self clean heat does not work.
I tried.
Maybe made is worse.
Easy Off Heavy Duty works but takes many tries.
I'm on sixth application and o 75% gone.
Putty knife helps.
Careful of the fumes.
I read that when sodium hydroxide reacts with aluminum it can be dangerous.
cooking.stackexchange.com/users/42127/JJ James
Unfortunately I can't post my pictures.
But I recently did exactly what he did here and with worse consequences of foil.
After reading everyone's post on several web sites, I tried the Heavy Duty
Easy Off Oven Cleaner.
After 4 applications and scraping with a "decal razor" scraper, I got nearly 97% of the foil off.
I let the solution sit for an hour on each application.
I was able to take the oven plate out of my oven and do this outside of the house.
cooking.stackexchange.com/users/43732/Tim Perna
Lining the bottom of an oven with aluminum foil or a foil liner is quite common and acceptable provided that the electric heating element is ABOVE the oven bottom, not below and that the liner is between the element and the bottom.
This of course is not possible with a gas oven.
With that said, you could try scraping the foil off with a putty knife, if it is not interfering with the operation of the oven, leave it.
Unsightly, but better than scratching up the porcelain finish of the oven.
The oven bottom should be removable and you could order another and be back to pristine in a matter of a few days.
Probably not what you wanted to hear.
If you are thinking of ordering a new bottom pan, you might get a friend to try heating the stuck on aluminum with a portable propane torch to soften the aluminum and maybe it can be scraped off with little to no damage to the underlying oven bottom thus saving you some money.
cooking.stackexchange.com/users/40590/trippt02
The Works toilet bowl cleaner from Wal Mart will dissolve it.
Put it on and let it sit for 4 or 5 hours.
What ever is in it dissolves the whole mess.
cooking.stackexchange.com/users/43280/user43280
I had this  brilliant  idea to put aluminum foil at the bottom of my oven while I cooked pizza at very high temperatures. I was hoping it would catch any toppings that fell off so I could quickly remove them and prevent smoke. The result is that the foil bonded to the oven and now I'm not sure how to get it off. Any chemicals or tools I should use in this situation?
How do I remove aluminum foil from the bottom of my oven?
cooking.stackexchange.com
<oven><cleaning>
cooking.stackexchange.com/questions/63280
cooking.stackexchange.com/users/7243/dpollitt
Easy Off Heavy Duty Oven Cleaner or other NaOH-based cleaners should dissolve it, but may take several tries. You could also try softening the aluminium with a propane torch, smearing aluminium/naval jelly on it, leaving it overnight and wiping clean before repeating if necessary. The Works toilet bowl cleaner  from Wal Mart will also dissolve it. Chemical solutions for removing aluminum stuck to the oven include solutions of sodium hydroxide, Works toilet cleaner and a propane torch. Naval jelly is another solution that seems to work. A chemical-free method would be to scrape the aluminum with a putty knife. The self clean heat option does not work. The easiest option may be to buy a new oven bottom.
Any solution that contains sodium hydroxide will do the job.  Alternatively you could try Works toilet bowl cleaner or a propane torch. Naval jelly works in removing aluminum stuck to your oven. Self clean heat does not work.  A chemical-free option could be scraping it off with a putty knife. Ordering a new oven bottom is another option.