Just guessing; is it possibly a photo cell to control the light in some way? although I don't know why it would be in the pool instead of by the controls.
Thanks for the responses... yes, the cover is removable. It screws on. It has been full of algae since I can remember. Because my pool accidentally drained over the winter, I finally was able to take it off and clean it out since it is not underwater. I think it may be related to fiberoptic lights that go around my pool that I never use. They are very dim so maybe this is just an indicator that they are on??? No idea.
Yeah, that's a fiber optic light lens. Even though they're dim, a good cleaning and a new lamp might brighten them up a lot. Do you know the make? there'll be a tower that houses the lamp and color wheel probably near the pad pr power panel.
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