I didn't want to remove it from the water, but in the interest of science I plucked it from the tank. It flipped its foot upwards and dropped its shell as soon as I picked it up. I guess it is a snail.
Cranberry: I am not sure if the chaeto was housed with corals, but I have added softies to the DT since I got this. The snail may hang in the weeds, but I pulled it off the glass. Here is the chaeto: