those are fly larvae. I ahve had that happen to me before.. for instance if you drop a fish into a bowl for a snake but the snake does not eat it, and the fish dies and you don't catch it right away, it doesn't take long at ALL for the only little fruit fly in your entire house to magically find that particular bowl and drop eggs into it. A lot of times when I am swapping bowls out and changing water if I have a soiled one I will set it on a shelf I have to be cleaned after i am done (I have spare bowls and i rotate in clean ones) Sometimes i will forget I put it there, and then this will happen sometimes... The amount of time the old dish was left to sit was MORE than enough time for that to happen.