When you shock it, I'd suggest going with a mustard algae level and maybe even trying some polyquat. I've had some similar weird issues with my pool. I suspect there are some odd strains of algae floating around.
I just did an ascorbic acid treatment because I had horrific stains. I didn't bother with the OCLT because we weren't going to be swimming yet, but the pool was holding chlorine quite well and was just slightly cloudy in the deep end. I took the chlorine down with chlorine neutralizer to do the AA treatment and then added polyquat to prevent an algae bloom. When I added the algaecide the entire pool immediately went milky white as though it was killing some kind of algae. It was freaky and very unexpected. The conventional wisdom here is that polyquat will prevent an algae bloom, but it's not very good at killing one that already exists.
I just did an ascorbic acid treatment because I had horrific stains. I didn't bother with the OCLT because we weren't going to be swimming yet, but the pool was holding chlorine quite well and was just slightly cloudy in the deep end. I took the chlorine down with chlorine neutralizer to do the AA treatment and then added polyquat to prevent an algae bloom. When I added the algaecide the entire pool immediately went milky white as though it was killing some kind of algae. It was freaky and very unexpected. The conventional wisdom here is that polyquat will prevent an algae bloom, but it's not very good at killing one that already exists.