you are probably looking at is a form of Clathrina ssp. sponge or cryptic sponge(low light) . they are harmless and come and go as water nutrients allow. a filter feeder.
sponges spread like mold using spores so you dont need anything in sump they free float and settle in areas then start to grow my old sump was the only place i ever saw them maybe 1 or 2 on underside of rock in back