He will move when he finds a spot he likes. Sounds like he likes it under that rock if he moved there himself.
Generally has to do with water flow and lighting conditions. I've a GTA that does the exact same thing. He just stretchs out into the light when he is ready. Drives me crazy since he is hanging upside down during the night. But if he is happy. Plus hermits have a harder time bugging him as well.
You could move your water flow devices around a little, but that creates other issues if they are already well placed.
If you "really" must move him then just move the rock. It's not too nice to go and try to pluck him off the rock and move him. Flip the rock over is all I need to do on mine but I'm just going to leave it alone for now myself.
75gal Salt, 18gal Sump, Skimmer, 60lb Live Rock, 90lb Live Sand, 3 Pwrhd, 2x55w Actinic, 2x55w 10k. Guest list: Goby Green Clown, Percula Clown and entire clean up crew.