Please help ID this 2-1/2" tall whitish benthic critter

[howardjg]

240g FO tank. 10 years stable. Newest 2 fish added 1 year ago. Tank: 12 common clowns, 6-line wrasse, Bicolor dwarf angel, sailfin Tang...
Several months ago these "growths" appeared. Grow to 2-1/2" high, attached to shells and dead-coral-rock. Spreading slowly, starting as gelatinous-looking blob (1/2") on rock or shell. ID ideas? Thanks in advance.

 

[Rtaylor]

Some sort of sponge?

 

[WheatToast]

Some sort of sponge?

+1. Generally are harmless filter-feeders and are part of a healthy reef ecosystem.