ive experienced problems like this in the past. fuckin package mediums are going to shit these days. not sure if its the best answer but ive always use hydrogen peroxide when i see shit like this. 25ml per gallon, a little extra ventilation on the root zone, and make SURE your shits bone dry between feeding. overwatering can look like a million different problems
That's mold, you may be watering too much or have high humidity. As long as it's not on your plants you're ok but it's a sign there is too much moisture. It may start growing on your weed so you need to tone down your humidity. That kind of mold usually appears on compost to help break down organic material.
Looks like what is known as cobweb mould. Make a very dilute solution of H2O2 and spray it. If it melts on contact, then that is what it is. It can grow like wildfire. Try upping the air circulation to reduce stagnant CO2.
Like the farmers above said, let it dry out in more between feedings. Also, you might want to look into improving you air circulation (intake, exhaust and circulation within tent/room) Although this is not mildew, it is a sign that you garden is definitely more vulnerable to it at its current conditions.
IMHO you are watering just fine. Don't go by how dry the medium is, just do what you have been doing and water when the containers feel light (about 50-70% lighter then when saturated, IMO).
If you let the medium dry out then thats the same as waiting til your dehydrated to drink a glass of water, its not healthy. Letting the soil dry will kill beneficial bacteria in your medium, also, the plants leaves will droop when they are under-watered, you want to water before this happens.
I know this is an old thread, but did you ever get rid of the mold?? Was your RH too high??