Bottom of pool - what is this part?

[Icek8s]

Anyone know what this square shaped form is on the left of the picture?
We’re assuming the circular part on the right is the main drain, but we don’t know what the thing on the left is. It’s in the deep area of the pool.
We’ve had a lot of issues with this area since the replastering was done a few month the ago. Today the plaster install company did the demo AGAIN to try to fix the issue and this was uncovered.

 

[1poolman1]

Right hand side - most likely main drain, but it looks like it has been filled with plaster. Left side - may have been a hydrostatic relief valve.

 

[Icek8s]

Thanks so much for the info.
We had the pool replastered a few months ago and there was an area that was disintegrating and discolored from the moment the pool started being filled. (See attached pictures.) We finally convinced the plaster installer to replaster the pool since the area was getting worse and the demo was today.
When the demo folks arrived today and they uncovered the 2 drain covers, and both were completely cemented over. During the demo, it seems like maybe they’ve uncovered one of the main drains, but not the other.
We’re wondering what needs to be done now to fix the issue.