Mesh. Thanks Texas. I am going to call some janitorial companies this week. They are closed today. Saturn do you close your pool?
With a mesh cover you need to close once the water temp stays below 60° and open before the water gets above 60°. That can be a challenge here since we can have long stretches of mild weather during the winter. This winter was really mild (this last week has been really warm!), so I'm not surprised your pool has already turned green.
Our pool, installed in 2004, came with a mesh cover. Usually, if I waited until late Nov to close (yes, I close our pool for the winter) and opened by the first of March there wouldn't be an algae problem. Also, mid winter I'd add another dose of polyquat algaecide. However, there were times, even with that open/close schedule, the water would be green at opening. Not only that, but I also noticed the mesh cover allows fine dirt through, making for a bigger mess to clean at opening.
After several seasons I decided to try to "convert" mine to a solid cover by covering the mesh cover with a solid tarp style cover. This has worked very well! Not only is spring cleanup much easier now, but I've not had a green pool at opening since doing this. I now close the first week of October and open the first week of May with no algae problems, even though the water temp is above 60°. As a matter of fact, a few seasons ago at opening the beginning of May I discovered our liner detached on one side and the foam behind the liner had bunched up all around the pool (probably floated when we got hit by two back to back Nor'easters with tons of rain while the pool was closed/covered). I called the installer (Aegean) to come fix it, but because they were so busy (apparently a lot of pools in the area were affected by the storms), they could come out until the 8th of June. I simply covered the pool back up and hoped for the best. When they came in June and we removed the cover, the water was still clear despite water temp around 80°! Needless to say I don't have plans to ever go back to just using a mesh cover.