Just spotted him when the lights came on today. He was fully extended then as the light stayed on he retracted back under the plug. Help with ID please
Look like either a gastropod (sessile snail) as they send out a little cobweb looking thin or a vermetid snail especially if it comes from what appears to be a calcerous tube.
Digitate Hydroid
They dont really harm anything. I have them in my system. I havent found an effective predator for them and manual removal is a pain in the bum. It feeds on particular matter in the tank but doesnt harm coral that I've observed
digitate hydroid 100%
I've had the same experience with them as Crabs McJones, but have read quite a few claiming they irritate some corals, maybe different species of them?