They are filter feeding sponges and will continue to spread. They are harmless and stationary
So, they're good to have? Where did they suddenly come from? I haven't added anything to our tank in well over 6 months.
Generally hitchhiker on live rock which favors low light and low flow areas
I have one of these, my guess was sponge as well. To those saying they are harmless, could they still irritate a coral due to proximity? I have one growing on my elegance skeleton, and I think it has gotten big enough to touch the flesh of the coral. . .
A very select few sponges could irritate corals due to toxins they produce (these sponges compete chemically with corals), but very few sponges seem to do this. Most sponges will happily grow along the skeleton/base of corals without bothering the corals or trying to grow over them at all (sponges generally are so harmless that even the more invasive ones can generally literally grow over the top of corals, and - as long as the coral is getting enough light still - they don't harm the corals at all even when growing over them).
They can spread and take over a coral in time if thats what youre wondering. I have removed sponge off my elegance as I want nothing on its wall. I have one recently discovered tunicate I found over the weekend allowed but generally remove sponges as I find them as they get too close to my coral
