Anyone know What this is ??

[reedwithE&Y]

5 month old tank, 20 gal reef tank. Just wondering if this is because my phosphate levels ?

 

[eggie]

Cyanobacteria they normally come out when there is an imbalance in the tank low flow as well

 

[Fish Fan]

I would agree with @eggie above, that looks like cyanobacteria. This is fairly typical in new tanks, I'd blow it off your sand with your trusted turkey baster, then siphon it out. Try to limit excess nutrients (nitrate and phosphate), and increasing your flow should help.

Good luck!

 

[eggie]

I think I see dynoflagellates as well in the rock.