Live rocks and dry rock has spots

[Zacksallhei]

My saltwater aquarium has been setup for 3 weeks and I have a triggerfish and 2 clownfishes and they are doing good as perameters are good. However I was wondering what this stuff is in the rocks and sand

 

[TX_REEF]

it is likely some algae beginning to grow. completely normal for new tanks. 3 weeks is very young, go slow and be patient with livestock additions.