Elisabeth, while in my friends greenhouse last week, I saw her douse the base of 1 of her plants with a spray bottle of rubbing alcohol... said she'd spotted mold, & the alcohol would kill it off! I've never heard of that before, but I figure she must know what she's doing, as SHE'S the 1 with the greenhouse full of lovely, healthy plants! (I won't say that I'm endorsing her method... just telling you what I witnessed! Considering the fact that mold spores DO go airborne, I will say that there doesn't seem to be a problem with any of the other plants in there... & when I was in her greenhouse again today, the plant she'd zapped with alcohol looked as healthy & happy as the rest!)
Myself, I would use $$$$, because it has worked for me on mold/mildew & black spot on 1 of my Rose bushes. I don't know HOW it works... it just does! (& it's QUICK!) Before I discovered $$$$, the way I'd handled sick plants was to totally uproot the plant, strip the old soil from the roots & give them a good rinse, then re-pot the affected plant in brand new soil & a new (or scrubbed/disinfected) container, after powdering the roots with Root tone. (I've never lost a plant by doing this, but have root-shocked a few that took a while to bounce back, cuz it's a pretty extreme method!)
I just now, happened to read your posts from Sept. ("prune hibiscus?") & it sounds like your poor plant has been in distress for quite some time! For this reason, I'm guessing that it may not have the energy to survive a radical replant just now, so I DO NOT recommend uprooting it! I DO, however, strongly suggest that you look into $$$$, as I think it may be your only hope at this point. (Unless you want to try the alcohol thing!) I'll PM you with details about $$$$, & I wish your Hibiscus the best of luck.
(Meanwhile, DO water it! Plants can recover from all sorts of diseases, & the mold spores are already in the soil... & aren't going to go away before your Hibiscus dies of thirst!)