Welcome
How about a picture of your plant? How long have you had it? Are you growing it outside or indoors? They like lots of water, so I doubt you could be overwatering. I have mine in just regular potting soil, so you might consider re-potting.
If outside, it's probably going dormant, as the nights have been really too cool for most Alocasias, I think. (you are only about 20 miles north of me) Nothing to worry about, it will jump up again once it's warm. Definitely bring it indoors or protect it for the next two nights that are supposed to dip into the 40's!
If indoors, again I think you might want to re-pot into good potting soil. If you plan to move it outdoors for the summer (they love it!) put it in a fairly big pot and wait until the nights are above 60deg. They are heavy feeders, so if you haven't given it any fertilizer, that's what it needs! Don't forget the fresh potting soil might have fert already added so check what the package says. IF it says something like "feeds plants for 3 months" then you can also give your plant a drink of soluble fert, something like Miracle Gro to green it up again, and give it a boost. After that, only give it soluble fert if it starts looking pale again. After the 3 months (or less . . . in warm wet weather) fertilize with a good balanced time-release product.
The white on the backs of the leaves was probably some sort of insect infestation, scale or spiraling whitefly. The soapy water treatment you used was just the ticket! Keep an eye on it, though as they might come back. Just wiping with a sponge with soapy water will usually keep it under control.
