I just bought a sea hair and it ate a little bit now buryed it self. I that normal? Or is it just taking a bathroom break? mine is huge and white and black spots.
I have heard stories of sea hares poisoning the tank if the die. However, 90G of water (plus your sump and skimmer) you should be OK.
It should surface soon. You should watch out for the powerheads though. They seem to get stuck in powerheads too often.