There are several banks in Muxia. Whether they change foreign currency I do not know. Google the two terms, banks Muxia and you get a list including ATMs located on a small map. Tomorrow, 2 May 2016, is a holiday in much of Spain and banks will be closed.
If you desperately need to change funds and the banks are closed try a large hotel.
There are banks in Muxía. And, AFAIK, May 2nd isn't public holiday in Galicia. OTOH, no idea if any of the bank offices in Muxía will change the currency you want to change into whatever currency you want to change it.
Most banks in Spain change currencies (not sure if just to clients or to anyone) so if you want to get Euros from one of the other main currencies, you have good chances of being successful (even if you give a hard time to the employees of the office if they aren't used to make currency exchange). If you want to Exchange Euros into other currency, you may find the office doesn't have the currency you want in stock and has to request it what takes time (i.e.: you wouldn't get it on the day)...