Sounds like diatoms. Is there an easy to clean dusty film on the glass and rocks, and in some areas hairy but when you pull on it, it breaks apart easily?
Diatoms from a new tank syndrome feed in silica from new sand and other sources found in new tanks. Yes the water you bought may be introducing phosphates that are making it worse, but nothing to panic about.
If you have filter feeders, diatoms are a great meal for them. Just wait it out. Google diatoms and you'll realise you're not alone.