If the grease in just on the surface, clean it off with a brake cleaner. If is has been of there for an unknown period, better to replace the brake pads (both sides). But you need to find the source of the grease. More than likely you have a bad grease seal on that side.
If it were mine, I'd pull the brakes on both sides, get the rotors cur, clean and grease the bearings, install new grease seals and you will have the rig set for the road.
Amateur Radio Operator (KE5DFR)|Full-Time! - 2012 6.7L Ford Crew Cab Dually -2013 HitchHiker Champagne 38RLRSB - Travel with one Standard Schnauzer and one small Timneh African Gray Parrot