Great Old One
Can you try to hide from a creature grappling you?
If the grappler can't clearly see you, i mean, it certainly knows where you are having you grappled, but the Stealth rules don't exclude it specifically. I use a similar approach to re-hiding in same location.
Technically, looking only at the rules, you are right, for example if you are in complete darkness, although he touches you, he cannot see you.
But honestly I would never allow it. He's touching you, so he knows where you are, and you can't move away.
I'm still trying to find a situation where it would really provide some advantage, though.