Black spots inside skimmer

[littlebigman]

2 month old pool. 15k gallons. VS pump. SWG. Oversized sand filter. testing and brushing twice a week.

I have noticed round black spots at the water level on the grout at the back of my skimmers. It brushes away easily but always comes back. No other spots anywhere. Sure looks like algae to me. Should I be concerned? How do I eliminate it once and for all?
FC never lower than 3. Would prefer 5-6. Getting used to SWG. just added borax today.
Any thoughts?

Thanks
Les

 

[frogabog]

A full set of test results would tell us more.

What test kit do you use?

What happens to the spots when you wipe/brush at them? Does an application of bleach clear them?

Have you used any trichlor tablets recently? They make my skimmer get a black ring.