what is this

Hi there, I have a hole in the pool wall around 6 inches below the water level. It is not an equalizer line and not connected to my skimmer line in anyway which appears to have one connection at the bottom going directly to the pump. See the enclosed pictures this hole has what looks to a valve...

Can someone please tell me what the heck this is and what to do to stop it? Saltwater pool and I find this stuff randomly sometimes in the pool. It feels like crystals but I don't think it's salt. I will walk by and see it, I'll clean it up and go maybe a couple of days before it happens again...