Mildew? Affecting lambs lettuce

Hi, my lettuce was in the greenhouse and started developing what looks like a mildew. 

as you can see in the photo, its affected most of the plants, i can rub it off with my thumb. 

Is there anything I can do to get rid of it. the soils is moist and I have just moved it outdoors to try and get a bit more airflow.

any comments welcome.

thanks

james