I saw this happen to an experienced reefer. He wasn't happy about it either. His prescription for it was just don't add any more fish or corals, just keep the water as nice as possible and soldier on. In a few months it all went away. Only wisdom I've got on the matter. Most bacterial blooms either get really bad or get better in a few weeks.
There's an extreme outside chance something like ChemiClean could get it, but I am EXTREMELY, VERY, MAJORLY hesitant to recommend that remedy to anybody with a young tank, especially someone who's a novice---because the effects of a massive dieoff of a bacterial sheet can crash a tank if the product is misused or if you don't have a powerful enough skimmer to handle the resulting dieoff of bacteria. If you do opt to try it, you need a skimmer rated for twice your tank volume or better, and you need to run it at highest efficency and expect your skimmer to froth like soapsuds, for days and days. I recommend it as a try ONLY if several weeks make it worse instead of better. Understand that such a treatment might still produce a tank crash, and you must have a quarantine tank standing by and ready, where you can put your specimens until the tank is ok again. I also caution you that I don't personally know if ChemiClean can handle this type of bacterial problem. As a technical note, there are 2 types of bacteria in nature, gram negative and gram positive. Your sand/rock bacteria are one type, and ChemiClean kills only the other type. This is why it works. But I do not know whether this white stuff is gram negative or gram positive. If it's the wrong one, the treatment won't work at all.
