I can only add this:
When you say "I just opened my pool" do you mean after it being winterized and closed for the season? If it happened during the course of a a whole "winter" (or whatever the off season is in your area), it could be a lot of things. Just about anything could have slid down the pool walls at those locations and caused it. Liquid fertilizers being used on your lawn by a contractor, some one sealing your wooden deck with a chemical product, acid rain. Did anything like that occur in the area of the pool while it was closed?
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