I'm making a Mythic Earth setting and I want to figure out how a race of human-like creatures could realistically grow grass or grass like hair. I'm basing the design of these creatures on of the Eloko/Biloko from African Mythology. They also have sharp claws and their mouths open mechanically like a snake to swallow food whole.
You could do it like this sheep did.
Have your creatures have a dense matted wool or similar substrate where you want the grass. Maybe gnarly dreadlocks. The wool captures dirt and moisture, your creatures are not too particular about hygiene and viola! Grass can grow.
This would be easier than some funky grass symbiont. I am not sure what sort of available nitrogen might be present in a hair substrate one a living creature - the breakdown of hair proteins might provide a fair bit.
Given the formidable nature of these creatures as you describe it might be nice to also have flowering herbs grow on them. A little yang to oppose the yin, as it were.
Some sloth grow algae in their fur for camouflage, and much more intimate symbiosis with algae is known in simpler animals like coral, so there is definitely a spectrum to work with. You could have them start like sloths but evolve tighter symbiosis with something like hollow hairs (like polar bear or porcupine) with a cavity giving the algae a more inviting place to live. The algae could start providing essential amino acids the eloko cannot manufacture itself much like the algae do in coral, you would have to come up with a mechanism of transfer (or handwave it)but the base of hair is well supplied with blood so it is not impossible, heck the animal could evel lick it hair for nutrients supplying extra water and maybe even nutrients for the algae in the process although this might not work for a human like creature but it could easily be a step toward direct transfer, blood to fluid.
I choose an amino acid becasue the algae would never be able to make enough glucose or any other macronutrient to matter but they could make an essential micronutrient like a single amino acid or a particular vitamin.