Looks like a nudibranch to me, check your corals and do a close inspection of any zoanthids to make sure theres no infestation. Some are harmless but others eat corals. P.s they can be very very tiny and hard to spot. HTH
Be very careful when you remove them from your tank as they hold toxins in their body from the zoanthids or paly's. If you squash them they could release the toxins back into the water or into you, then you could get quite ill. I had them once and used tweezers which were thrown away afterwards.
cheers for that lucky used tweezers and then squashed them in a pot but i never new they are toxic.. good job i never killed them in the tank then. i hope they have just come on my new coral i bought the weekend and i dont have loads of them in there
Most nudibranch diet are sponges and in tank sponges are hard to suvive as well. So most don't last long even though there are really stunning colourful ones.There are 1 or 2 that can survive long and are algae eater but are not easy to find.