Yes, the green came fast and furious, and family wanted clear water now. I found out that I can drain and refill in just under 24h. Before draining, I cleaned out the filter sand with the hose. I believe the hollow ladder steps with holes in them are probably the algae headquarters. Those will now see regular attention until we decide on a replacement.
So, now I am following TFP directions, but need a little help because the test kit won't arrive for a day or two. In the meantime, all I have are test strips. So the pool was drained to about 4" deep, and I started refilling. At that time, I dumped in 1 gal of 12.5% chlorine and a sock with 1.75# of CYA (chart said 15,000 gallons need about 4# CYA to get up near 50) A couple of hours ago the pump could start again, and added 2 cups cellulose. After 20m of filtration I tested the water and the FC was at or above 10, the CYA was under 30, and the PH was 7.6-7.8, TA was OK. I only had about another pound of CYA, so I crushed that in a sock at the return jet. We swam for a bit in the freezing water (92 degrees outside though).
I just tested the water again and FC 10+ with the CYA very close to the 30-50 color, slightly fainter... perhaps 25? But need to check in the morning. Question: what else do I do tonight? She's not crystal clear since there was still some of the old green water, so probably risk of turning again?