If you know you have MRSA on your leg, then you should also have some MRSA cream for that which you can use on the spot on your arm. You should also talk to your doctor about ways to prevent spreading MRSA to other parts of your body or infecting your child(ren) or SO. MRSA is not to be taken lightly and is very contagious.
If its MRSA you should have been on an antibiotic for MRSA. Not all antibiotics can treat it, that's the problem with MRSA. In fact that's what MRSA stands for, Methylin Resistant Stalhylicoccus Aeurus. You need to have the Dr culture it, then most likely be on IV antibiotics. MRSA is not to be taken lightly.
I don't know for sure, but I suspect that a cream that you rub on it won't do much. Go to the ER or urgent care right now before it spreads more, and get antibiotics that will help. The sooner you get treatment the better.
You never get rid of MRSA, but if it is MRSA you need to go to the dr for treatment and make sure that you are washing everything in hot water to kill the bacteria. And washing your hands A LOT. Or using hand sanitizer.