How do I clean my truck's cap?

I bought a white fibreglass truck cap and it's plenty dirty. It has a lot of green moss like stuff and what looks like tiny flecks of rust.

I have a pressure washer, but I think it's gonna' have to be scrubbed by hand in places. What can I use and what can I not use to clean this thing up?

Thanks!

Fiberglass is pretty tough, I assume its gel coated?

Start with regular car wash and a brush, then look for a product called "GRUNT". It might be available from a marine supplier. It will clean up any stains etc from the rust and green stuff.
If you can't find it, then a non-abrasive (no cutting compound) cleaning/polish compund will work.
I know the mothers brand does a very good job.