Citrox is good against green algae and is completely harmless to plants and the environment. I use it to scrub seed trays and pots. You can get it from the Organic Gardening Catalogue but it would be worth checking other sources too. I add Ecover washing up liquid to the Citrox when I wash pots
For our tunnel, we use a pressure washer, but you need to make sure it doesn't splash back onto the plants - maybe you could cover them. A small pressure washer is surprisingly cheap and of course can be used on vehicles too.