Pool uncovered all winter/Ph question

[let_her_flicker]

Have a couple of quick questions:

First tested the PH and it is higher than 8.2 This was taken before I added any Chlorine. Is it possible to get a false high PH? I have added some M. Acid and color changed somewhat (I dont see reds very well).

Next, took Cl up to 20 last night and dropped to 10 by sunrise. On top of the water I noticed a light foam. This started after adding CL to water. Is this something to be concerned about?

I know next year i will start adding chlorine to pool water in February to prevent a green swamp that I currently have. It did go from dark green to an aqua green in one night.

Thanks