In this case the fireflies are actually expected. See this guide:
https://coronarenderer.freshdesk.com/support/solutions/articles/5000529356What you can try to overcome them, other than what is advised in that guide:
- Using less intensive and less sharp light source (e.g. overcast HDRI instead of Corona Sun)
- Lowering the overall reflection glossiness of the grass material (something as low as 0,3 - 0,4 should work)
- Using Corona Denoiser in "remove fireflies" mode
- Using Highlight Clamping in Render Setup > System (not Highlight Compression in the VFB) - but remember this will kill the high dynamic range in your image if you need to save in 32 bits
- Rendering in double resolution and then reducing