everyone has alternate takes in reefing for sure Im opposite here
not that this is bad, but there are sponge-wrecked posts and they got that way by alignment of variables.
Try and make it gone from one area of your rock, even though its not bad now
if you can't, then you know what variables are aligning. my own reef turns 15 in december and Im simply unwilling to chance anything. I had a acan frag for about 6 years, a prized one, even made it into a magazine shot photo but on its underside was a similar sponge that never, ever left this one frag.
but it would envelop the whole polyp and Id have to clear it back
even after rasping, and 35% peroxide spot-applied to the rasped areas, the sponge came back for years. solid
unbeatable
the rascal was given away free to someone with eighty warnings, who still wanted the frag. it was them or the trash.
one day in June I decided it just cannot stay, associated with it is the most virulent sponge if it took over my reef that's takedown material. I could not beat it with any form of care.
try and remove it, see how quick it comes back.
i give you a 99.% chance nothing is harmful, my live rocks have had normal sponge succession over the years. it was just that one though, it was too mean. too risky, my tank is too old to lose it/took total universal command --> booted gone