Just opened my pool this evening and upon removing the cover I noticed my skimmer box covers (2) are stained brown. In the pool the skimmer door seemed ok but the rectangular finish piece around it is stained as well. Water was below skimmer for the winter during closure. Pool water is nice and clear. There were a few leaves on the bottom but overall it looked pretty clean. Any ideas what could have caused this? How to remove? I don't have any test readings yet as the sun went down before I could do any. Will do them first thing in the morning. Any help would be appreciated.