Well, to start of with natures, I wouldn't say there is no best nature for the Alolan-Grimer, but there are some good ones. Since your Grimer will eventually evolve into Muk, I will base my answer off of that.
Muk's best stats are Atk and SpDef, and worst stats are SpAtk and Spd. Using this, there are 4 combinations (one + and -) you can make for your nature with one of the most favourable in my opinion being Adamant if you want to play offensive, and second being Careful if you prefer defensive. The other two Brave and Sassy have -Spd, but it may be good option to you since majority of Muk's attacking moves are special, and none of which I believe have Spd to play an effect on.
For the items, you may choose to use the Assault Vest or Life Orb if you natures Adamant/Careful and you don't plan on using status moves - also because Muk does have a good HP stat. However, if you do plan on using Muk's large status move list while stile have, you can simply use the Shuca Berry to weaken it's ground-weakness.
Your Choice! Hope I helped! :)
