Cleaning a fibreglass roof is straightforward especially if this is done periodically. Safety precautions and consideration of others may need a bit of extra thought and preparation.
Remember you are dealing with a roof and the almost mandatory dangers of height - Even just a garage or porch height can be very dangerous. Accidents happen out of the blue, that's their nature. Going at it 'Gung Ho' will just increase the risk.
Think about, and plan, what you are doing, be diligent and careful yourself, and think about any affect on third parties and their safety.
People don't look at roofs really, even their owners, unless a problem arises. Your cleaning shows that perhaps you are ahead of the general curve so use this opportunity to run a quick check for damage or anything which might affect water-tightness.
First make sure that your access (maybe a ladder) is to a safe and secure standard and take appropriate precautions for yourself and any possible passers by. Your fibreglass roof will stand light foot traffic (assuming the supporting woodwork is of appropriate strength) but be sensible and cautious how and where you place elements upon it including yourself.
You will need some really basic cleaning elements
A stiff brush & possibly a second clean stiff brush
Warm to hot soapy water in a bucket
A pressure washer is an alternative
Fibreglass roofing cleans readily if regularly maintained -
Sweep the roof clear of any debris, leaf fall etc. (Make sure that this is removed responsibly, not just thrown over the edge as this will cause blocked gutters or worse a danger to passers by)
Use a clean brush to wash the roof off with warm soapy water
Swill down with clean water sensibly.
Check any gutters for debris or blockages.
Note - wetness gives rise to slippage - be cautious and clean near any open edges from a position of safety - plan this .
If you have one and it's safe to use you can use a pressure washer. You shouldn't need this on full blast. Just be careful that you don't flood the guttering or if you have adjacent wall trims and flashing don't blast water up and under these. Of course the spray (including dirt) may fall on other property - please be considerate.