How can I remove the mold on the roof of my outside building?
A bleach solution will remove it. You will get the best results if you use a power washer.
What type of material is your roof? Some types of roofing cannot be cleaned with a power washer so check this out before starting. Also, working on a roof, especially a slanted roof, can be very dangerous and if you are using a power washer it should be done from the top so as not to damage your roof.
I would recommend that you get a cost estimate from a reliable company and then decide if it is worth taking a chance on doing a job like this yourself.
You may need to replace the roofing because once the spores are established, the mold can easily come back, especially if the conditions for its growth remain. Cutting off the trees are a good step.
Power wash with a bleach solution. Why does it have mold? You will need to correct that or it will come back.