It's not at all uncommon for stagnant water sitting on plaster to cause a "bathtub ring" effect. This is why we caution people against draining a pool because if you leave any water in it for any length of time, you can get a ring. Your plaster is basically brand new and it is, to a certain extent, still curing and hardening. Leaving that little bit of water at the bottom was more than enough to cause a difference in hydration between the wet area and the dry area. The water also probably had plenty of mineral hardness in it so that any pH rise would cause minor amounts of scaling to occur. Believe me, it takes exceedingly little amounts of scaling, less than what the naked eye can see, to cause enough surface changes in the wet area to produce a "bathtub ring".
Time and low CSI water will cause the stain to gradually lighten but it may never go away fully. You got bad advice and bad service from the people working on your pool and that's frustrating for sure. That light could have easily been changed FROM THE DECK SIDE as all lights are often changed that way. This is why lights are set no deeper than 18" below the water line - so a guy, on his stomach, can reach the retaining ring by simply lying down on the deck. The contractor that was demanding the pool be drained simply wanted to have it his way so he didn't have to strain himself a bit. Changing a light is a two man job if you want it to go easily and I'm guessing the guy that did it did not want to pay for the extra man to be there. It's ridiculous but it's also water under the bridge at this point. You can complain all you want but I sincerely doubt you'll get anywhere with either the PB or the tech that changed the light. The ring can go away on it's own if you manage your water properly.
