What is happening to my terrain?

So recently i came back to unity because i wanted to make a game with the new editor.... it was pretty confusing but i fixed the problems, but this one is just weird.
There are dark patterns on my terrain. I tried putting light, but it fixes it for a couple of minutes before it comes back with an different weird pattern. It also dissapears when i come near it, but comes back when i get far away. Yes, it does appear while playing, and not an editor bug. If i put more light, the grass looks really light and unrealistic. How do i fix this problem??? Here is a screenshot of this pattern :
https://imgur.com/a/vf5Q4Qf