First off, rubber trees love humidity and spraying with water should be fine.
I was going to say it looks like some kind of fungus, but if the leaves are sticky it's probably some kind of pest. Aphids and spider mites are the most common ones that secrete a sticky substance, but I don't think either makes damage like this.
Hopefully someone else will have a better answer, but no matter what quarantine that plant. I'd also treat with a spray that kills both fungus and bugs (something like this https://smile.amazon.com/dp/B000BQP5I0/ref=cm_sw_r_cp_apa_fabc_co-PFb0M4D0A7)