Oh those are coralline algae which is what you want on your rocks. Coralline algae will outgrows the nuisance algae. The healthier the coralline algae growth, the less likely the nuisance algae will grow! Lots of coralline algae is a sign of a healthy environment with good water parameters in your tank!
But if your water parameters aren't good enough, even you put in a rock full of encrusted coralline algae they'll slowly die off. So maintaining a good water parameters is a must! And good lighting too!
You can buy some LR or see if any reefers giving out LR and mix with dead rocks, the dead rocks will eventually become "live" too!