Algae again?

[cschnurr]

Three weeks ago, upon return from a two week vacation, I returned to algae in my pool, which I cleared up thanks to reading up on this forum.

Last week I was away from the house for another week. Just before I left, I increased my SWG ouput to avoid a repeat of previous problem.

Yesterday when I returned, the pool was green again.
Before I did anything, my numbers were

FC: 7.5
TC: 7.5-9 (No noticeable change in colour, but I am diluting the sample to read above 5ppm so readings are not exact.
CYA 55
TA 100
CH 380
Backwash water looked relatively clean.

So I've boosted the Chlorine up to 20ppm,
I will monitor this 3-4 times per day, with regular brushing, vacuuming and backwash. I know I'll get this cleaned up in the next couple days.

I have a couple questions about should I have done anything differntly.

1)FC at 7.5, how could the algae form?

2) Is it possible that I never completely got rid of it? even though my numbers were good, and the water looked clear.

3) I thought it could be repeated because of contaminated sand in my filter. So I drained the filter below the sand line. It looked pretty good to me, but I did notice that the first gallon or so of water was greenish, and then the remaider of the drained water was clean.