That is water behind the structure. The area behind the light should be excavated and a water proof material should be applied. I would also water proof the interior as well.

The water source should also be re-directed via piping to keep future water away from the shell.

The gunite around the light may not be thick enough.

If water is coming in, it will certainly go out once the pool is filled. It's a misnomer that the plaster will seal and water proof the inside of the pool.

If I'm correct, it looks like a membrane was applied to the raised area and the area where tile is to be set.

Thank you for your response. There is definitely water behind the structure. This area is super heavy clay. They dug the pit with a skid steer rather than an excavator, so they dug themselves a ramp down into the pit right behind that light fixture. There was a little bit of water standing against the shell when they back filled their ramp after the gunite cured. And then the heavy rains came. My concern now is that they've now built a natural clay funnel to draw water right to the shell behind that light fixture. Picture of the earthen ramp below.

You are correct, they did apply some sort of membrane on the area to be tiled. It was a liquid they applied that has a rubbery texture.. Tile and stone is going on as we speak.

Clay soil is expansive soil which means it holds water which can affect any concrete structures including home foundations.

A perimeter drainage system carrying water away from the pool to a collection point would be helpful. This would entail excavating around the pool wall, putting in gravel and a perforated drain pipe.