Forcing an app to clean itself? I've been swiping around the home screen on my iPhone when I see the text under the Facebook logo switch to cleaning..., but it's nothing that I made it do, is there any way to tell an app to clean itself?
I found this OS X Daily article that should answer your question.

Basically, when storage is low, iOS detects apps that take up a lot of space with old caches and temporary files and automatically delete them. It's perfectly normal. The user can't force iOS to clean apps, though.