Denoisers are an imperfect, but useful tool.
I have this scene where there's light coming in through a window and I've given the room a volume to catch the light. If we run a low amount of samples you can see the fireflies produced.
This is normal, the nature of a volume with lower density is that it will only affect some rays cast from the camera. The denoiser gives us this ugly result:
with only little wisps of dust visible. This is also normal, because the denoiser can only work with the information we give it. I only ran this at 12 samples, and it is really interesting that it can produce this result.
Usually the answer is to run more samples, but even at 300 samples it comes out as this:
Which is part of the way there. I cranked a small region up to 1200 samples to turn this:
into this:
That's a lot of samples, but you can probably see from the noisy image that it would take many thousands of samples to render this volume in a realistic and pleasing way. Volumes with lesser density are just very noisy. The grass will be a lesser case of this, where more samples will give each pixel more influence over itself.
I did find in doing this that volumes aren't contributing to denoising normal or albedo, probably by design, but I wonder if it isn't related.
You may want to play with Adaptive Sampling instead, run some experiments with a render region.
