Are hydroids bad?Starfish, and either hydroids or a sponge. Not sure
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You don't want hydroids. They are probably sponges tho. If the tubes have nothing coming out of them, they are likely sponges.
That 1st one is an asterina star. Bunch of different species, some munch on coral. I used to pick them out religiously until I realized I had never seen one on a coral. Now I leave them alone.
So I just noticed the tubes a day or 2 ago, they seem to be on most my rock work... [emoji15]I agree that the thing with the tubes is probably a sponge. But I'd keep a close eye on it and see if it spreads. A sponge will grow slowly and hydroids spread randomly.
The star is an asterina, and like rockhead51, I used to pick them out off the front glass when the population first bloomed. But it settled down and I've never seen them bother a coral. Even if they are on a coral, as they move off there is never any damage. IMHO the coral eating of asterina stars is way over blown!