These look like diatoms to me. One case is that if your tank is still fairly new, diatoms will almost certainly appear, but it will go away after some time. What are you using for your water changes? Tap water? Diatoms thrive in the pressence of silica, so if you are using tap water in your top offs or water changes, chances are, diatoms will continue to remain in your tank.
Your photopeirod is too long. You may want to consider reducing it to eight hrs.