Is it safe to store an open beer in a sealable cup in the refrigerator? I was given a large can of beer. I couldn't finish it so I poured it into a sealable coffee cup and put it in the refrigerator.

Is it safe to drink?
The (relatively) high alcohol, presence of anti-bacterial compounds from hops, and low pH of beer make it inhospitable to most micro-organisms.  

Given, it's beer, and cool and sealed: Yes, it's safe to drink.

But it will be flat!