Every now and then, depending on the "lighting" I see what looks like dirt stains on the bottom of my plaster pool. At night with the pool light on, the bottom looks beautiful, but other times (cloudy outside) it seems like I'm seeing stains at the bottom. Bright sunny days "hide" this phenomenon as well.

Is this just an optical illusion from the cloudy skies? It's wierd, I know, but it looks like brown stains. Maybe I need a vacation?