This is an adult Vertigo pygmaea from the backyard. Their shells grow to be about 1.8 mm long. These snails have appeared in several previous posts (for example, here and here). I always find them on rocks. I had been suspecting that they grazed on the thin films of green algae that grow on those rocks.
I collected about a dozen of them last week. After I measured their shells, I put the little snails on a little piece of tree bark inside a little cup and then sprayed a little bit of water on them. The bark was covered with green algae. I have noticed over the years that the captive individuals of this species don't remain active for very long even when their surroundings remain wet. Sure enough, about a day later, all the snails in the cup had become dormant by attaching themselves to the bark or to the piece of toilet paper underneath, which was still damp.
That little stringy thing next to the snail is a clump of snail feces. Its dark green color indicates that the snail had fed on the algae on the bark. There were several other similar clumps. Now I know that at least the captive Vertigo pygmaea eats algae.