Yeah, I would have a few concerns about the weight, roots, and water drainage on top of the retaining wall. It really depends how the wall was built and the strength of the wall both to vertical loads down but more importantly to horizontal loads pushing the wall outward. I think problems will not show for a few years out.
I have retaining walls around my pool area that are about 20 years old and some needed to be rebuilt as the wall began leaning. My walls are not directly along the pool like your is but along terraced areas.
Only time will tell if your builder built the wall to handle the load the trees add to it.
it's actually quite difficult to tell how far the trees are away from the wall in the picture.
If the wall was properly constructed with the proper drainage behind it root intrusion or weight shouldn't be an issue. I would really hope a wall built on the pool beam was constructed correctly. It's all the other things I would be concerned about. I can certainly see why you would want such a beautiful screen behind your pool however I think you will be paying a high price with all the debris in your pool. Probably would have been better if they could have been quite a bit further away from the pool. Then there's always the chance of the wind snapping one off or blowing a whole tree right over onto the pool or wall and damaging one of them. Something like that would probably be many years down the road.