Could be water boatmen also, much preferred over backswimmers because they don't bite. I seemed to have 1-6 water boatmen randomly everyday in the past 2 months. Somedays 0. Surprised the heck out of me when we got them. Didn't realize that water bugs could be an issue (1st time pool owner here!). We also have ponds about 100 yards behind us which is probably a major factor.
Anyway, I guess the key is to remove the food source (algae primarily) and they hopefully will fly to some other source more suitable to their needs. So be sure you've done all you can for that.
2 weeks ago I went to borates at 50ppm hoping that it would help. At first it didn't seem to, but yesterday, out of the 6 I fished out, 4 were pretty much comatose hanging on the walls. Also, today, while swimming around checking things out I found a couple of dead ones on the bottom. I was very happy to see that!
Chuck