Pressure washing would be a good start to remove the bulk of it IF the water isn't going to penetrate the observatory and cause damage.
Following that I'd guess most mould killer spray ons would remove the smaller stuff, or white vinegar in a spray bottle, along with most bleaches. Keep in mind bleach and other remedies may kill the growth in the immediate area (grass, vegetation).
https://www.bunnings.com.au/30-secon...-away_p4460169