Recurring Algae on Bottom of Pool

[16X32AG_Bob]

I have a 16 X 32 AG with 6 ft deep center area and have an AutoPilot SWG.
Chemistry as follows (PS233):

CL > 5ppm (OTO only as DPD Reagent R0871 is on order)
PH 7.2
TA 80
CH 150
CYA 38
Salt 2270 (via AquaCheck test strip. Yes - a little low)

Filter = Sand

I typically run the chlorine a little high, but, whenever I leave the pool covered for several days(5-7), the bottom begins to turn green. First a light coat of algae forms and then if left for a couple more days the bottom begins to form heavier “streaks” of darker green. All this while the water stays very clear. Of course, once the kids enter the water and stir things up, the entire pool turns green and requires re-shocking, extensive filtering, and so on. I purchased the SWG in hopes to avoiding blooms like this. What am I missing?

Help! Bob