All nuisance algae are caused by excess nutrients, period. Sometimes it takes quite some time for a tank to mature to the level where all the nutrients can be processed. High quality live rock is key to better nutrient processing.
The PhosGuard would help (and did), but a stronger product would work better. Any of the rust-colored products are better than the white products.
It's really difficult to try to guess why you're getting this algae issue (could be diatoms or dinoflagellates from the description), so your best bet is to take a read through the algae link in my signature and come back with some questions after that.
