sponge or macro algae or ...?

[sspiff]

Thoughts on what I've got here? It's about the diameter of US quarter, and as thick as maybe two quarters stacked on top of each other. It is solid but pliable, kind of like a piece of silicone tubing or a gummy bear. The very edge of it (~2mm worth) will gently flutter in the flow. Entirely on the sand, out in the open (exposed to light). Has not moved from this spot since we noticed it. Thanks!

 

[EnterName]

Cyanos sometimes form blobs around stuff this way. Depending on what's in the center it might explain size and overall consistency.

 

[sspiff]

Thanks! Well, it's current consistency will make it easy to remove!

Another interesting bit we've noticed over the last two days: overnight, something digs out around it, and the morning finds in sitting in a shallow depression with bits of sand on top. By the end of the day, the sand has been cleared from its surface. Flow doesn't account for the sand movement. Other tank mates include: pair of clowns, bristletooth tang, some hermit crabs, some trochus snails, and a rogue gorilla crab that we haven't managed to catch yet. We haven't seen anything near the blob during the day.

Pic from this morning with lights almost at full: (now this evening it is free of sand)

Two days ago (mid-afternoon):