Yellow Chalky Splotching

What causes this splotching on the pool walls?  It is easily brushed off but shows up again a couple of days later.

The pool water was just changed last month.

Pool is in ground with sand filter with Arizona location.   Thanks.

That sounds like mustard algae. Brush it off and shock the pool. It should go away after that.

Will do.   Thanks for the help.

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