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!