fungus on Japanese Maple
This is a lichen. They are very common in Western Oregon's moist forests and landscapes. Lichens can grow on trees, but also grow on rocks, soils, and mosses. These beautiful species are actually a combination of a fungus and an alga. The fungus shapes the lichen and the alga photosynthesizes to feed them both.
Lichens have rhizines, which are little projections from their bases that grow into tiny cracks in the bark so the lichen can adhere to the tree or another surface. Rhizines do not enter the living tissue of the tree and as a result lichens do not harm, or steel (act parasitic) nutrients and water from the tree it lives on.
So to answer your question, yes leave it on the tree. It is beautiful and safe for the tree. Lichens usually also indicate good air quality, so it it is doing well then there is a good chance the air around your tree is cleaner too!