Welcome to TFP!

To make sure we understand, you are looking to install something on the top of that 2-ft brick wall to keep something from falling down the slope into the pool correct? If so, are you focused on people/pet safety only, or are there other concerns such as dirt and debris?
paticus said "correct" but I'm not sure you two are saying the same thing. I think paticus is talking about a net that stretches
across the pool, to keep his kids from falling into the water, not one that goes vertically behind the wall like a backstop. Do I have that wrong?
If I am right, yikes, I can't offer any ideas. You need to be able to walk all the way around the pool and spa to connect/disconnect the net to/from its anchors, which are typically located in the deck, several inches, maybe a foot, back from the pool's edge. Is that the type of net you're considering? I suppose you could anchor it to the wall, but then you'd have to be In the pool to disconnect it to swim, then reconnect it before you get out. And that would get old real fast. But even if that were possible, you'll have some amount of gap between the net and the wall, like a scalloped edge. Hard to say if those gaps would be kid proof or not. Technically, 4" is kind of the standard for that, as in: if a 4" sphere can pass through an opening, then that opening is not child proof.
You'd have to push that wall back several feet, two at least, possibly three, to achieve the amount of flat deck you'd need. That's a lot for that slope.
Is there a way to install a fence around the pool? Is that an option? Even if there is a way to rig the net, they're a pain: taking it off and putting it back on for every swim! I'm too lazy. I'd rather just open a gate and jump in...