Brown lump on acan frag

[uninformed]

Any idea what this brown lump is at the base of the frag? I haven’t pulled it out yet to scrape it, but it has growth over the last month or two based on pictures.

 

[uninformed]

I found a couple bristleworms hiding under the plug (not the lump), but I still can’t tell what it is after scraping it off. It’s firm, not squishy or jelly-like, and kind of fleshy, almost like squid or octopus meat. It’s definitely not rocky.

 

[eggplantparrot]

Probably a sponge of some sort