I wonder what your CYA was when you closed?

How are you testing? CYA tests with methods other than the disappearing dot test are not trustworthy.

Regardless:
1. Add 2 gallons of bleach at night. Doing so will gradually consume the chlorine demand, and should gradually oxidize whatever residue you have.
2. Keep the pH high, for now. Low pH tends to result in some pretty nasty chlorine+goo compounds; high pH tends to favor less nasty versions.
3. Do NOT cover your pool. Some of the compounds likely to form are removed by chlorine + solar UV, rather than chlorine alone.
4. If you aren't using the K2006 or an equivalent drops based kit with the DPD-FAS chlorine test, you need to get a K2006.
http://pool9.net/test/
5. If you have any idea what your CH (calcium) level is, say so.
6. ID your filter by make / model.


Unfortunately, cleaning up the remnants of bio-degraded CYA can be an extreme PITA. Good luck.