Some tiny insect larvae that hatched on the side of the bathtub (yuck).
The eggs for these larvae were most likely laid in a moist green scrubbie, which was covering a small puddle of water. This is probably similar to the way that some other insect eggs are laid in stagnant water, usually near rushes or other vegetaion that will slow the water even further.
Moral of the story? Make sure unused sponges (etc) are hung to dry properly!