My apologies if the answer is obvious, but I’ve not seen these before. They don’t seem to fit what I’ve read about hydroids. They have doubled in number in my 10 gallon over the past month, popping up in different locations around the tank. They behave almost as some sort of tiny tube worm or duster, retracting back into the rock if touched or disturbed. Sometimes the clowns will pick at them. Fully extended, they’re about 3-4mm in diameter.
Further apologies for the low quality photos!
Further apologies for the low quality photos!