What The Heck Is This?

Tank is 6 months old and all of a sudden this thing poped up out of the sand yesterday. Looks kinda like a worm tube but not sure. None of the fish, crabs, etc. will have anything to do w/ it, and there is a lump under the sand 'bout 2.5 inches across, perhaps 1/4 inch high. Might just be a hill of sand...or might be something else. The two things we have that stay under the sand right now are a horseshoe crab, and a moon snail.

Any ideas? good or bad?

...kinda tough to get a clear picture w/ our digital camera...but you can tell it is "J" shaped w/ a hole at the end. Whole thing is maybe 1.5-2.0 inches tall, same color as the sand, and as thick as a pen refill.

Looks like some sort of siphon. My nassarius snails have them but I don't know that they get that large. Maybe put some gloves on and dig it up with a spoon to see what's under the sand there.

Ok...after two days there were three of them. So I spooned them out of the tank today. Nothing below them. They were just hollow tubes made from sand (see pic where one crumbled into a pile of sand). One had a brown substance in it that appeared like fish poo (see pic). Could these tubes be created by the moon snail?