Thanks i have dipped him, nitrates could be better sitting around 20, he is growing, haven't seen anything eating him, but it just saw a big hairy looking crab, that i didn't know was in thereHi Susan,
It could be anything .
I assume your parameters are ok ? What are they?
How long has it been in your tank , Have you introduced any new corals?
it could it be Montipora Coral-eating Nudibranch
Might be worth looking into a dip for it?