What is this white build up

[Mccoyver]

My EPDM liner in my garage pond has a white slime collecting on it and it seems to be clouding my water up as well. Anyone know what's this is? Should I be worried? Should I clean it off? My water parameters are great and I do 30% water changes weekly.
Thanks

 

[Sarertnoc Kram]

Pics


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[Mccoyver]

Don't know how to post them on here. It looks like it could be an algy but I have had lights off for a couple of days with no change.