that is a porcelain crab. live in anemones especially carpet anemones. i have one living under my torch coral. it is peaceful, filter feeder. and may occasiaonlly eat bigger pieces of food. willl not harm the clownfish. however, if a clown fish is hosting anemone, the crab may choose to live there too. then they will compete for space but the crab usually ends up losing.
no worries. harmless and a great addition to a reef tank