OK the mimic mechanic is pretty simple, it just appears as a chest once you reveal it, which is a positive thing, because it gives you a 2nd chance not to fight it if you don't have the health for it. In other words, if you see a 10+ lvl and are afraid of the mimic, you can safely click, and as long as there's no lvl 11, you will get at most lvl 7 (or 8 if you didn't discover pink slimes yet).