Can moss cause roof leaks?

Yes it can.   Roofs are constructed to channel water down the roof and into the gutters.  If there is too much moss on the roof it can block these channels and eventually cause leaks.  A heavy build-up of moss should be removed and the roof treated with a biocide to prevent re-growth.  However, you should never use a high-pressure water to remove the moss as this can damage roof tiles.