Its not hydroids. PGreef has a pic of it on the first page of the thread. Its grey detritus looking gha. Could even be thin gha with detritus stuck to it. Anyways.....I've had the algae on some of my rocks and it never spreads to the lit areas of the tank. Stays short, thin and on underside of rocks. Does not seem to be an issue as it doesn't spread or grow like a nuisance algae.