Mean grass is a mob that looks like real grass in the game, but if you go right next you it, it grabs you and slowly eats your heath away. They spawn in any biome containing grass and on any grass block. For every minute, theres a 2% change of natural spawning. If you attack them, theres a 20% chance another one spawns nearby. They drop seeds.
I don't think this fits into vanilla. We haven't had any "hidden block" enemy yet, and I think that's for a reason. Especially grass, which is abundant. Grass is supposed to be a safe block that you have around your house, but with mean grass you'd always be scared just walking around in daylight. Mean grass making an appearance might scare younger players, and you'd never have grass outside your home because that invites mean grass to attack. Lastly, "mean grass" is a silly name. How about a rare mob that acts as a sort of chameleon, becoming one of the nearby blocks? It makes more sense as a transforming monster than not nice grass (which even if it was mean shouldn't be able to attack you, yes even though its a game this bothers me for some reason).
A 20% chance one spawns nearby if you hit it? How much health does this thing have?!?! And all I get for killing it is seeds, an item I can get more than enough of if I grow a tiny wheat farm. Not only that, but it has a 2% chance of spawning every minute? Is that per-grass block? Even if it wasn't, that means during a 1-hour play session I'm likely to have spawned at least one of these, just imagine the pain servers would go through... Or people doing AFK sessions!
I get offtopic a lot, be warned. I also often mention a Minecraft clone called Minetest. The clone is nowhere near playable with it's default game in my opinion, but it's at least the start of a good open-source voxel rendering engine, and maybe one day it might be decently playable.
I think this idea could have potential, it just needs some revision. If instead of being single pieces it spawned in clumps that were approximately 7x7 blocks, there was a visual (and maybe audio) indicator that you were near it before you actually got hurt, and the spawning if you kill one was removed I wouldn't mind this really.
Maybe it could be like a natural Creeper hangout area, so Creepers who aren't actively trying to murder you have a tendency to group inside it. Then if it visually reacted to the Creepers you could say "okay note to self, bad grass over that way."
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First of all, there are a few hidden block mobs. Such as Shulkers, but they at least give off sounds and have a different texture than the blocks around them. Also, this thing would spawn way too quickly, it would fill the plains biome, once the most peaceful place in the entire world, with terror.
Maybe have it be Crab Grass, and make it a clumpier, uglier grass, with darker tips. It should have a rarer spawn rate, and it should only spawn in dark oak biomes. If you think this is unfair because of the Mansions, those really only spawn very far away, and are overall not common. There's new biome-specific mobs like the husk anyways. It shouldn't have a lot of health, and certainly shouldn't spawn another, or at least not with a 20 percent chance. a 2 percent chance would be better.