Why are my plug impatiens going mouldy on the top. Is it too much watering?

Yes, it needs to be allowed to dry slightly. Try putting them in a warm spot, and they may survive.

Otherwise, it might be a fungus. I'm no expert but try getting them to dry out anyway, all the best!

improve the ventilation and if possible water them from the bottom. Mine are sitting in a tray that i water from the bottom. they are in the greenhouse but near a vent. so far so good.