Anytime a Hoya feels stressed (overwatering, underwatering, sudden temperature change...) they drop leaves. Yours is suffering from mildew from lack of air circulation and maybe overwatering.
Hoyas don't bloom if the light isn't bright enough but no direct sun.
I would spray with a fungicide, and find a brighter spot with good air movement. Consider repotting into orchid bark mix with some cactus soil mixed in. Your current pot isn't the best as it's not allowing for proper drainage and drying. Choose a pot just barely bigger than the root ball - my favorites are terracotta as they are heavy enough to keep a growing hoya upright and also allow quick drying.
