brown diatoms to green algae question

I have a green algae question.

I have a fairly new reef tank. I got the normal brown diatoms that lasted
just less than a week then it all turned green.... and the green grew fast.

Is this part of the cycle? I read that once somewhere, but I cannot find
any other reading supporting that brown diatoms, which I know isn't algae,
give way to green algae.

The algae is powdery or slimy I guess. It is not hairy and does not come
off in sheets; in fact it doesn't want to come off much at all. I have a
handful of snails but they aren't exactly thorough so I don't think buying
tons more of them is the fix.

Some stats:

90g
20 lbs dead Carib Sea sand and 10 lbs live sand (can't remember brand)
18 lbs dead base rock, 55+ lbs of live rock
actinic and HD lights on about 10+ hours a day on a timer, moonlights
thereafter
2 emperor 400's
hermits, astreas, elephant slug, few tiny feather dusters
ammonia = 0
nitrite = 0
nitrate = 10, was 20 a few days ago
pH = 8.4 steady

I haven't done a phosphate test but I'm betting it's part of the problem.
Why? I hate to admit that I did use tap water in the beginning. I treated
it but I was uninformed. Less than 10% of the water is RO/DI now. And who
knows what was hiding in my store-bought sands and salt.

Anyone have experience with a UV Sterilizer?

Thanks for your thoughts.
Bryan
www.geocities.com/bryg30/ - for a pic of the mess