Need ID of stuff on Rock

[Dr RBG]

I have this rust color stuff on some of my rock. It is hard and does not scrape off. What do you think it is and what should be done, if anything. The tank is about 7 months old and is otherwise doing well (450 reefer) Alk is 11 dKH, phosphate is 0,07ppm, Nitrate is 20 ppm.

 

[Diesel]

If tank is doing well and parameters aren't out of whack don't worry.
It can be form of algae or discoloring of your rock.
You know rock will look totally different in a year or so.

 

[Dr RBG]

Thanks