Start by looking for any bus service PLA to Masca www.titsa.com Without looking myself. you may have to go the long way round Santa Cruz-Puerto de la Cruz-Icod de los Vinos etc.
There are many agents that do trips to Masca in PLA and at the harbour in Los Cristianos. Maybe you could book one and do a one way, then find out ways of getting back.
One ferry company that operate from Los Cristianos is Fred Olsen
Another is Armas I think
Had time to have a quick look, and I think you need the 460 from PLA Estacion de Gua Gua pronounced Wha Wha, to Los Gig, then change to the 355 to Masca.
Best to double check Im right.
Buy yourself a Bono ticket (12 euros) at the Alpispa shop, departures end of Reina Sofia airport, for discounted bus fares. Or at bus station PLA or ticket booth Los Cris terminus.