Again, without seeing pictures from YOUR pool, I can't be sure, but MOST eyeballs screw-in, and most 'flanges' are either part of, or retained by the eyeball.

But . . . I'm not going to tell you to unscrew it, without seeing a good picture of the actual eyeball & 'flange' on YOUR pool.

Also, it's not too likely -- unless you've seen reason to think otherwise -- that the leak is from the pipe into the concrete. More common is a leak from the *pool*, along and around the pipe, through the wall, to the ground.

I re-read your first post, and realize it's not clear, so I'll ask. Is water leaking OUT of your pool, causing water levels to drop, OR is water leaking INTO your pool, bypassing the eyeball?