Easy enough to find out, pull the wheel and take a look. My guess is it's a wheel cylinder. I was fleet mechanic for 30-years and I "never" saw a front one leak grease enough to run down a wheel. rear ones yes but not front. Now that I've said that it will probably be a grease leak. If it is wheel cylinder leaking and it's not pitted, I would hone them, put new cups in and it will good. It's okay if there are pits in the center where the pistons and seals "do not" ride. You probably have a better chance of rebuilding them then buying new ones. All the new stuff is junk anymore. Nothing wrong with rebuilding them, we used to do it to save money. We finally started flushing the brake fluid every two-years and that just about eliminated the wheel cylinders from going bad. We had 400 vehicles, and we kept them a minimum of 15-years. You are supposed to change the fluid every two years because it attracts moisture, the moisture causes rust in the cylinders.