I found 4 of those things in my tank a few weeks ago
. The first one was basically an adult and just chilling on the filter. It was white and I'm assuming was drying it's wings. I found the other 3 younger ones hanging onto the vallis at the top of the tank over the next week. I have removed them all but I keep expecting to find yet another one. I'm just glad I don't have anything smaller than neon tetra in that tank.