It sounds like a sponge. You might try using a small diameter
steel straw using something as a gasket to use is as a nozzle in a 9/16" diameter siphon hose and siphon it out. Reason for using a small diameter nozzle in a large hose is to increase the suction at the end of the nozzle but it will clog easier so some experimentation may be needed to find the right size to remove the sponge without hurting your acro. After siphoning out as much as you can use a syrenge filled with H2O2 with a long needle and gently squirt small amount (1-2 ml) into the remaining sponge. This is a very gentle operation and be conservative at first until you know what your acro can tolerate.
Fortunately it doens't look like a
black Terpios spp. sponge which would aggressively encrust your acro and smother it. I've only seen it once in an LFS display aquarium and it was a serious pain to get rid of and some colonies had to be tossed.
