DPD to check pH?

I used both the old and the new OTO kits and they matched...

It also seems that I cannot keep chlorine in the pool, but I've never run the pool with such low chlorine before (I really don't want to bleach out this new liner). I did just peruse Pamsel's post re: her experience with a new liner and keeping chlorine as well. However my TA is around 140; CYA 50 now.

Last weekend I swapped out a cheap new solid winter cover that within hours of putting on began tearing apart at a seam (lesson learned -- never buy a cheap $50 cover, they're not worth it). I bought a high-quality (expensive) cover from my liner guy and it's much heavier and doesn’t even have seams like that cheap one. But, this is all to say that my cover has been in place and when I viewed the water last weekend while taking the one cover off and putting the other one on with the help of my neighbor, it looked as clear as could be - in fact it could not have looked any clearer. So I find it really hard to believe there may be algae. Water temp I'd guess is around 78.

With the old liner that I didn't much care about because it was entirely bleached out, I would shock to 15 before I put the winter cover on, and over the course of 8 months the ppm would only drop about 11 ppm (to 4 ppm when I uncovered in May). I'm in the south so I don't close my pool and the water does not freeze. I just run the pump on the handful of nights that it drops below freezing to safeguard the above-ground pump pipes.

With the awareness that a high ppm of FC would be sitting in that new liner month after month, I did not dare shock the new water. Each day it seems I put in enough bleach to bring the ppm up to 3 and a day later, it's gone. My liner guy said I don't need chlorine with the cover on, but it makes me unsettled. Can it be anything other than algae? I just can't bring myself to shock it now with the cover on because I don't want the high ppm in there for months on end. Any ideas? Or should I just not worry about it with the water cooling, winter on the way, and the winter cover on?