I have read very mixed opinions on these guys as far as shyness. I think I am more on the lucky side, but I got to pick him out and he was out in front of the tank at the store, which seemed to be some kind of indicator. (I was not actually planning on buying a Moke because of the hit or miss shyness, but he was calling out to me!)
He is also nice and green. That being said, he's still shy in that if someone walks quickly past the tank or if I get up on him too fast with my phone camera, he'll dart off. But he's not been in the tank long, either - only a month. Also, he prefers to hang out near where there are plants at the bottom of the tank, so he has a little cover.
He took to pellets after maybe a week of feeding them after a fasting day, and soaking them in thawing bloodworm cubes. But his favorites are definitely the fish and krill, or live earthworms.
Would a 20" ray be able to eat a 5" Moke? They are not girthy like the Dels, more narrow bodied like a Teug. Him at 6" is a totally different shape than my Delhezi at 6".
I just got a 4" Palmas Polli from Wet Spot and that little guy is ALWAYS visible and he's only been in the tank for not even 48 hours. If you dont' want a hiding poly, you might choose a different one to assure you see them, like a Senegal or Palmas Polli or even a Delhezi. All three of those are known to be a bit more active. I also read about Ornates being active.