I saw one in my pool once. Apparently they feed on algae, which I wasn't aware I had. So I shocked the heck out of the pool and I've never seen him again. I've read that if you see them, fish them out and squash them dead, don't let them go. And yes, I would think that the problem is coming from next door if you're absolutely sure you don't have algae. Do you have a lot of them? I wouldn't put the soap in the pool. It would take a lot, it seems to coat the top surface and I'd not be happy to deal with the possible fall out from that. Good luck!