Can't answer specifically for Tenerife but Halloween is not celebrated as such in Spain. It is the following day which is a religious holiday - Dia todos los Santos (All Saints Day) and also known as Dia de los Muertos (Day of the Dead). It is a day for remembering deceased relatives and many thousands of people go to cemeteries to lay flowers, tidy graves and spend time there with their families reflecting. To give you an idea of how important this is, in the Alicante area where I live several hundred additional bus services run on 1 November for this very reason.

However with the all pervading American influence it is not uncommon for there to be Halloween parties and trick or treating. When I first moved to Spain this was pretty much unheard of but has gained popularity over the last eight years.