Power wash the deck, and reseal or restain it. BUT, before you do this, prune the tree. You'll need to prune it back a bit more than from just the 'perimeter' of the deck. I'm sure if the wind blows, the sap will get 'blown' onto the deck. You may need to prune back quite a distance.
What type of deck do you have? Trek, pressure-treated, redwood, cedar???