Looks like red algae or cyano bacteria, if the tank is 3 weeks old, has it already cycled ? was it purchased as live rock ? it's a common pest in new saltwater tanks, what are your water stats ?
It takes about 4 months to cultivate enough bacteria in your bb colonies to partially remedy this. Cyano Bacteria doesn't really attach well so you should be able to siphon most of it out, or use a long turkey baster to pick at it. Because it feeds on nitrates and dissolved CO2 you need to perform water changes to ensure you flush the components it needs to grow out until your tank is fully cycled.
You may also have the wrong bulbs, your tank looks very white, you could be feeding it the wrong wavelengths which will aid the growth. Are you running any kind of a skimmer in your setup ? that could also help isolate the problem and prevent it from showing up in your tank.