I'm not sure if there is a direct bus from Finisterre to Muxia but you can get there by changing bus in Cee (20 mins by bus from Finisterre).
Use the Monbus website to get bus schedules for each part of the route for the particular date you have in mind. As an example for next Wed, 4th July you could leave Finisterre at 11:45, Arrive in Cee 12:05. Depart Cee 12:45, Arrive Muxia 13:21.
Bus schedules do change depoending on work days, weekends, public holidays etc. so you would need to check your date.
Alternatively, if you are passing through Santiago prior to going to Finisterre, the Galician Tourist Office in Rua Do Vilar (just down from the Pilgrims' Office) will be able to provide you with information.
This site is run by Ivar at in Santiago de Compostela.
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