Certified GKH Gardening Expert
Can you include a photo? This sounds a bit like a few things.
Powdery mildew can appear as described, as could salt damage from chloroamine buildup among other salts present. These solids will not evaporate.
Downy mildew could be the culprit too, but this one may be more detrimental to your plant. A photo will help me to pin this down.
In the meantime, this article will help you with the proper care of these plants: https://www.gardeningknowhow.com/houseplants/ti-plant/growing-hawaiian-ti-plant-indoors.htm