Are these black dots or moderately size smudges of some type? Or maybe they are gray to black lines. With the latter, consider turning the leaf skimmer over and using the other side. The aluminum metal ring that holds the net is making those marks.
The first is black algae and must be treated with an algaecide for that purpose. Read the labels first to determine which will work for you and follow instructions.
The other type described would be the results no doubt of an imbalance in chemicals. Check your Ph readings and correct soon.
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