Crab molts can look like the real thing. I actually had a clown fish die in my 10 gallon before. He died at like 2in the afternoon, I assume, that is the last time I saw him.
At 5, after not having seen him for 3 hours, and I was sitting next to the tank most of the time, I started taking the LR
out of the tank. I ahd to go throug hteh rocks twice to find him. He was dead, and was copeltely covered by a sea slug. I managed to remove the fish from underneath it, EVERYHTING BUT THE HEAD had already been devoured, bones where not even attatched anymore, there was a bit of tendons or somehting that had part of his tail still connected to his head, but in a time period of less than 3 hours, this slug had taken care of almost and entire clownfish. I did not see the slug for another 4 months when my tank crashed, and he died and came out of a piece of LR
when I removed him, so basically, you could have one, and he may hop onto a dieing fish (my clown was headed towards death at the time) and take care of him quickly. I dont think he could get onto a healthy fish though