Hello Abbothayes88,

Welcome to the boards. I couldn't enlarge your pics much, but from what I can see, it surely is a mould, especialy if a dehumidifier is helping with the situation. Heater at night is a good idea to help dry air, but you can still use the dehumidifier at night if you don't need the heat. Humidity levels increase during the dark time due to the plants transpiration.

Mould is a air born pathogen carried by spores. Since you've had mould in your grow area spores will likely be there. You need to clean your grow area with a mild bleach or hydrogenperoxide solution....all surfaces; walls, cieling, around your lights and fixtures, every nook and cranny.

Good luck with it and good growing.
OM