The game heart my eyes

If you are able, try adjusting the sharpness levels on your monitor little by little. The defined edges in grass and trees would hurt my eyes over time, and make it really hard to focus.

I think i had something similar FIRST time i played this game. But it must be the movement of youre character and because there is much to look at and not the screen itself.
Also make sure you play with lights.

I sometimes have this problem but I think for me it’s caused by dry eyes - at least my optermist said that when he checked my eyes out they were extremely dry.

if it is grass it might be shadow flickering that is most prominent on moving grass.

You may want to try lubricating eye drops. I have very dry eyes and this helps me a lot. Not the harsh eye drops for removing redness, just get artificial tears.

I would make sure to play with a decent amount of light around you. My eyes get bothered if the screen is bright but my surroundings are dark.

Perhaps adjusting the gamma correction in the game display settings may help.

Most people don't play video games in a healthy lit environment unfortunately. Monitors are always factory set on very high brightness levels and most buyers don't even bother to change the brightness settings. Also playing on a dark room with a very bright screen is very unhealthy. Lower the brightness of your screen around 50% or even lower if the monitor is bigger than 24" inches or sits very close to your eyes. Be sure to have some sources of light around but not directly facing your eyes. A good lighting behind you or hidden behind the screen can help. It depends on you and your environment on how you need to adjust lighting. Also be sure to stand on the right distance from your screen. Sitting to close is bad as it is sitting too far. You don't want to stress your brain trying to read tiny letters or burn your eyeballs by sitting too close. Be sure to visit a doctor as well. There may be a health problem lurking. Try some tinkering with your monitor and lighting first.

If you are on windows 10 at night, try turning on Night light, click the bottom right where the notification bit is, it softens the output.

If you're noticing it from grass or trees, maybe it has something to do with marginal screen tearing? Smaller, patterned objects and distortion over them can be more prone to causing less than obvious eye stress. I used to have that issue with Tera and Rift, where small distortions in bright areas of terrain due to screen tearing would stress my eyes.