It is normal, your cover lets sunlight in. If you wanted you could start adding bleach and circulate it with a small pump before you take your cover off so when you open your pool it is clear.

Really though it does not hurt anything, and you can clear it up in 3 or 4 days.

If I could do it over I would get the solid loop lock cover, the one that has a pump to drain the water off the top.