yeah those are definalty sycanoid sponges... from
SpongeIDFAQ2
"these critters are very common and commonly misidentified or "unknown" incidentals that develops in marine aquaria with live rock. They are Syconoid sponges of the genus Sycon. Filter-feeders... harmless, and even desirable. They can reach plague proportions if the tank is neglected or has other nutrient control issues."