Tazzywazzy, your plants will still be 'breathing' even though it's cold, this is condensing out on your polycarbonate. You need to ventilate your GH when it's warmest (lunchtime, maybe?) and pick off the mouldy bits so it doesn't spread. If you leave the lid/doors open, cover the plants with newspaper or fleece if it's colder again or they give up. They should start to recover in a week or so.