You have powdery mildew and it's not good it must've gotten to humid when you went away... well It's symptoms usually appear after only 3-7 days and it will spread in a short time. Dense plant growth and low light intensity are favorable for it to do real damage. High RH is favorable for infection and conidial survival, but infection can take place at RH levels as low as 50%. Dry conditions are favorable for colonization, sporulation, and dispersal.
As solution goes you could move it to the sun or get more lights, also try to give them space. Air flow and ventilation will discourage mildew growth. Also Powdery mildew thrives where high rates of nitrogen have been used and you do have dark leaves so try to ease it on that.
You MUST Prune infected plants to get rid of infected parts and increase airflow. If the infestations are severe, KILL the plants that are infected. remember to disinfect your pruning tool in a bleach solution of one part household bleach to four parts water after each cut. Watering plants in the morning gives the plants the rest of the day to dry off, discouraging establishment of diseases, including powdery mildew.
Sulfur is highly effective against powdery mildew if used in a protectant program with a minimum of 7 to 14 days between applications. Garlic naturally contains high levels of sulfur and a few cloves crushed in water can be used to make a homemade spray. Apply a sulfur-based fungicide at first evidence of mildew and repeat applications as necessary. Proper timing of fungicide applications is critical to successful control so make sure to begin at the first sign of the disease.
Another option is to spray once a week with a solution of baking soda. Baking soda increases the surface pH of the leaf making it unsuitable for the growth of powdery mildew spores. Be sure to spray the undersides of leaves as well as the upper surfaces when using any of these sprays.
Here's a recipe to make your own spray:
1 teaspoon baking soda
1 quart water
A few drops of liquid soap
Before treating your plants, test the spray on a few leaves to make sure they are not too sensitive.
Credit to Outdoors at GC