My sister keeps seeing these at her Key Largo Beach area.. at forst she though it was a brittle starfish, but they stay in one place... as she is curious but skeptical of picking it up sh has noticed that if you get close to them they disappear into the sand...
Old fugger with a bad heart, who just wants to fish
That was my first guess too... but they don't move and if you touch it or get to close it sucks itself down into the bottom like one of those sea worms or anemone. Starfish just crawl along the bottom, brittle stars kind of move like an ocotopus in different directions.
If it was me I would have yanked it up by its roots to see whats attached, but my sister is a little nicer to nature.