What is this?

sponge

yes this is a sponge for sure, u can tell by the tubular protrusion called the oscule and by the pigmented nature of the membranes, keep a good flow if u want it to thrive!!

I have something (a few actually) that looks like this in my tank, but it shoots out little "spinerettes" that filter feed in the flow. At least thats what I assume is going on.

hey captain, are the lil spinerettes like lil star legs? i have more than a few.