We also own a 27AG and at the moment it also looks green and swampy. We never cover it in the winter. I did leave my leaf net on too long and it froze so I just took that off and some of the leaves feel back into the pool. My plan of action will be to get those leaves out first, vacuum to drain all the green slime on the bottom(so that none goes to the filter). Use brush on the walls and then let the filter run a good 24 hours, then do water testing to see what I will have to add to water.
This is our 3rd summer with the pool and every spring it looks the same. It is amazing how quick it will clear up! Very important to brush the walls down though. You want to get rid off all that green scum on bottom and walls.