yeah thats why I said sponge first basically basing it off your description of how you found it. I said monti because you can in no way tell what it is based off the pic. Could be sponge, could be monti, could be epoxy, no idea. Need a clearer white light pic.
Its most likely fine as long as its staying in the dark. Usually beneficial. There are types that will grow to plague proportions. Yes a clearer pic may help someone to tell what species it is that may give better answers. I have had white, yellow, red and a purple sponge show up in my reefs in the past. The one white one and the yellow were very invasive growing into the light and into or on top of corals in its path. They would kill the corals they grew into. Another white one would die in the light and didnt seem to affect anything negatively. The red and purples seemed fine also but would grow into the light. The grew around zoas and really most other corals not harming them more like they could not contend with the corals. Usually its a sign of good biodiversity.
