Is the greenhouse completely closed--no fan, no ventilation from the roof, no way for air to get in and moisture to escape? If it is totally enclosed, either replace a glass pane with a ventilation pane (looks like a big Venetian blind that you can open and close) or get a fan and leave the door ajar for two or three hours a day with the fan blowing out.
I envy you your greenhouse. I had a small one when we lived in town, but now on the farm the winds would take the poor thing down, so no greenhouse here!