I have an 18X36 ~25000 gallon in-ground vinyl pool that has a mesh cover. I've only owned a pool for about 3 years now. Still learning.

My problem is every year I open the pool it has a massive algae.

I have two questions.

1) What is the best way to clear this up?
2) Is there anything I can do to help prevent this in the future?

Thanks for your help.