Ive had these turn up in a tank before from stones collected in my local stream. I assumed that the stones had eggs of some sort on them that i overlooked. Not positive though.
I started seeing fry disappear in a planted tank, used for growing out fry, then on day it was sitting next to the front glass. Each day the school of fry diminished by at least 2 or 3 .
It's a guppy breeding tank. Water temp heated with one of those pre set heaters. I freaked out and threw in in my native tank. A pumpkinseed ate it immediately.
I have the same in my guppy/shrimp tank. I actually found a damselfly sitting on my tank light the other day newly transformed. No idea how to fix it was my tank looks like the amazon rainforest overgrown ×1000 lol