Chlorine dropped to zero

There is more than one screw on the light fixture it looks like there may be 4 or six. Can't get to it. Might have the guy who opens and closes my pool to do it. Everything else is pristine. Wouldn't the chlorine at 24 ppm get into the light fixture and kill algae ? I'm so upset with this. It's almost July and I'm still trying to get my pool right
 
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