Too damp, not enough air flow probably, so moss is forming, or algae or whatever it is.

Yup exactly! You need to have air moving through the greenhouse all the time. Given how wet it has been recently the humidity in the air means more ventilation than usual.

moon growe, what would i do without you...

many thanks also bamboo..

I have the same problem. Defo made the mistake of leaving my greenhouse closed for a couple of days, when I went back it was so hot and humid in there.
The plants will be ok, just take the algae off. The soil might be quite dry underneath.
Defo need good ventilation, and not over water
:-)

Tinman how are you watering your plants? The pots seem to be simply sitting on a wooden surface.

moon growe, Built myself a cloch with a wooden base..
water them when they need it (rain water).

So how does the water drain easily from the pots if they are sitting on a piece of wood? You need to get the pots into trays of some description and water from below not above and don't let them stand in water at all.

Bad as it sounds you could also try watering with tap water. The chlorine in it stops the algae from growing whereas in rain water has the spores in it.

Owdboggy's right - recommendation from the RHS is to water seedlings with tap water.

Yup I agree with Owdb. too