Is this algae or sand/dirt?

[brainfreeze]

we’ve had a lot of rain/wind over the last week.
Ran the dolphin robot last night, pool was clear after that and today morning I see this.
Is this dirt or algae? Had added shock last night as FC was at 2 ppm (cya 30)

 

[mknauss]

Do an Overnight Chlorine Loss Test
Raise the FC to 15-20% of CYA level for the test.