You can't get at it to clean it off.
I had all my rear lights replaced last week under warranty because of this problem. Take it into your dealer and report it to them.
Access to the moss is too difficult, so this probably wont work.Not read everything on this thread, BUT go to a horse place and get some Horse Shampoo(yes it sounds daft)make sure you ask for the stuff with Anti Algae stuff in it, it certainly gets rid of/kills moss on soft tops.
I know Robbo, Ive seen the small tolerances, but if your car is out of warranty, it may help someone, or even as a preventive measure to stop it coming at all, I've not seen where it is exactually, but I'll guess just under the clear lip where you can always see the bubbles after you've washed it.Access to the moss is too difficult, so this probably wont work.
Best going to the dealer