Hydraulic brake not retracting sufficiently off the disk?
You should indeed start by just loosening the caliper mounting bolts and trying to do a simple adjustment of the brake. Try to center the gap either by eye or by squeezing the lever while tightening the bolts back down, which sometimes works great and other times not at all. Tighten the bolts to about 8nm, which is firm but not super tight.

If that doesn't work, remove the wheel, remove the brake pads (careful never to touch the pad surface), clean around the pistons with alcohol and a rag, then stick a tire lever in and push the pistons firmly back in place. You've now reset the automatic centering of the brake. Put the wheel back in and re-adjust the brake. You have to work the lever a couple times until it firms up.

If none of that works, you probably have a sticky piston. If it's fixable, it's done by re-lubricating the piston seals with Shimano mineral oil.