I had the same issue. I bought the pool brand new early spring of this year and it had the aerator valve. It didn’t leak for the first 2 months of using the pool. Then I noticed a consistent stream of water dripping down from the valve everytime I turned my pump off. I originally thought that maybe that’s why we have the little black cap, but it didn’t make sense because my pump had a timer, so it defeats the purpose if I have to be present to plug up the aerator everytime my pump would automatically shut off. So the leak was definitely not normal.
I ended up taking the aerator off the valve, and looked for any blockage or damage. Turns out I had some debris stuck in there. After I removed the debris with a small screw driver, I pushed some water through the valve with a hose in hopes of flushing out anything else stuck in there. Put the aerator back into its spot on the inlet valve and voila! No more leaking!
Hope this helps!