Typical usage would be 2 - 3 ppm per day. But it can be much higher with incipient (barely visible) algal growth.

There is a bit of green spotting on the liner so that could be it.
You have NOT reached chlorine levels that would reliably exterminate mustard algae. Typically, that requires levels equal to 30% of the CYA level, or more -- 30+ ppm in your cause -- PLUS brushing.

Chlorine consumption also goes up, somewhat, as the water temp rises.

And, there are any number of pool additives which will create a long term increase in chlorine consumption: any product from United Chemicals, most products specifically for mustard algae, most stain removers, and plus other products we haven't specifically identified yet. Have you used any of those?

Anyhow, if you would, fill out these two forms, so we can be more confident we're not overlooking something:
http://pool9.net/pf-equip-form/
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