Sounds like mustard algae; keep your chlorine ABOVE 3.0 100% of the time.
If you switch get a DPD-FAS testkit, so you can actually *measure* levels above 3, you can probably fine tune it more than that. (We call them 'guess'-strips for a reason!). Check out the test kit link in my signature if you are interested: get either the K2006 OR the K1515. And no, they aren't available locally.
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