Mysterious brown flakes? Algae? Mushrooms?

[Cromulent]

Relative newby with a mystery. What is it? Brown small flakes growing on rocks.
Fluval evo 13.5 g, 7 months old.
Nitrates (5-10)
Nitrite 0
Salinity 1.026
Phos barely discernable
Alk 10
Calcium 420-450 (varies with water change)
Temp 77F with inkfish controller

Two small clown fish, snails, starfish
Murder crab (been killing snails despite empties available)
soft corals, few LPS, no SPS.

Have dealt with aptasia, long hair algae and a recent light green algae (lowering hrs of light, more frequent water changes)

Thanking you in advance,
Crom

 

[PR_nano]

Picture looks like flatworm. Compare against Google pictures. Might need to suck some out first before treatment.

 

[JGT]

Padina algae.

 

[DanConnor]

Another vote for flatworms. Not a huge problem; many fish will eat them- wrasses in particular

 

[Cromulent]

Thank you everyone for your help. I used a piece of tubing and siphoned most of them up. Will keep at it.