I've never sprayed it indoors but I would imagine it should be sprayed with the lights off. Either an hour prior or just at lights out. I've only used it on outdoor cannabis plants, my vegetable garden, and roses. I had no PM last year when I typically get bad PM on some of my vegetables especially the squashes and cucumbers which are a PM magnet. The roses didn't get it either like they do every year. There has actually been a significant amount of study regarding the US of potassium silicate and PM.
I also wouldn't spray it on the buds as it's going to dry on the plant but if used throughout veg it should build up on the leaves to create a barrier preventing PM from taking hold. Although, after thinking about it some it would probably wash off with a bud wash at harvest like people use if their plants had PM during flower.
"Potassium silicate (KSi) impose suppressive effect on fungal growth and increase plant immunity by forming a physical barrier on leaf surface (Liang et al., 2005). KSi enhance the concentration of free amino acids and defense related enzymes in the plants"
"Spray of nutrient solutions before the onset of disease, result in significant reduction of disease incidence."