Looks like a Polaris universal wall fitting (UWF) for a pressure-side cleaner. Can you feel water coming out of it when the pump is running? Do you have a booster pump also?
EDIT: I'm probably wrong because your fitting is threaded on the inside. Never mind.
It's opened all the way through, but it's not sucking water when the pump is running. I have a pool cleaner, which is connected to the skimmer. This is the pic of my pump system. I also doubt it might be a connection for a pool cleaner. Maybe it's just not hooked to my pump?
It looks like someone removed the threaded wall return eyeball fitting and screwed in a 1-1/2” to 3/4” reducer. They may have put some kind of custom fountain or spray attachment there. It looks @JamesW located an old cleaner line.
If you feel like digging around the pad you can see if that old line is 1-1/2” PVC below grade. In which case, that wall fitting can be converted into a suction side cleaner if you hook that deprecated PVC line at the pad up to the suction side of the pump.
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