Would normaly say just scrape/brush it off but not everybody is wanting to move hook ladders about etc etc
Pressure washers are very popular although make sure it is done from the top of the roof down, to reduce the amount of water that may go through. Also may cause lots of damage to the tiles!!
Could try spraying any of the normal fungacidal washes etc and then either let nature take its course or jet it off, giving the fungacidal wash time to do its magic first.
Liquid Plastics Bio-wash is very good and would be my first choice:
Not cheap though.
Go for the clear or your roof will have a nice blue tinge to it, although the blue with gold guttering maybe the next big thing