White patches spreading on Calamondin leaves? Please help.
I'm fairly new to keeping houseplants, so I'm not terribly sure what is wrong with this plant at all; I found some grey/white dusty looking patches on some of the leaves late last week, (and something very similar on the Pitch Apple I have sitting next to it) so I separated them from the rest of my plants. The clusia is downstairs, the calamondin still in my room where it was growing before.
I thought it may've just been dust settling on the leaves, so i wiped down the pitch apple, which is fine now, but since the calamondin's leaves are so small and difficult , I just inverted it over a sink of lukewarm water and swished it around a bit before letting the leaves airdry. It's definitely not dust on the Calamondin, but I'm not sure what it is exactly?
Since bathing it on Thursday evening, I've tried wiping down the particularly "dusty" leaves each day with a damp paper towel, but it only seems to be worsening? It spreads to more leaves each time, and is now appearing on the underside of some of them. I'm not sure what to do to help it; from what I've read, all I can think of it being is powdery mildew, but I'm not sure at all, and I'd really love some second opinions and possible suggestions on how to fix it!
The room it's in is normally around 18/19 C, it sits on the windowsill during the day, so it gets plenty of light and the soil drains well. I keep eight plants in my room normally, but this is the first time I've seen this happen to any of them? I've had the calamondin for around about 2 months now.
I've severed all the fruit and dead leaves from the plant and isolated it from the other plants (for fear of whatever this is spreading), I hope that's enough information? If I've missed anything that'd be useful, please let me know and I'll try and provide that? Thanks for your help.