Looks like diatoms to me. They come and go. If you make a drastic change to your system, including adding or losing fish, coral etc, you can set them off. They usually resolve themselves in a few weeks.
If you are interested in learning about reefkeeping or have a problem with your reef, our reefkeeping community is here to help. Feel free to ask a question or search our site. We have lots of experienced reefkeepers that are willing to provide free reefkeeping advice!
Besides being a great resource for all levels of reef aquarium hobbyists, CaptiveReefs is a social experience that will enhance your enjoyment of reefkeeping. CaptiveReefs is committed to connecting reefkeepers with the support and information they need to grow beautiful coral reef aquariums.