It could be something that the nursery sprayed on, as in a pesticide but it is most likely caused by a sucking insect such as aphids or mealy bugs. After they suck sap out of the plant, the excrete a sugary substance onto the leaves or nearby plants. But I can't figure out why you couldn't rub it off; did you use water?

It has been suggested to me that the Tibouchina might be sweating, and that this is the residue left from it. No sweet taste to it. Does any one think this may be a viable answer?

I have the same problem - sugary looking substance that won't rub off. Mine is dropping leaves and buds, but still blooming. Any help out there?

Pictures of the problem would definitely help here. I've never seen such a thing with Tibouchina growing here in California, although I wonder if it could be hard water deposits from spray irrigation inside a greenhouse? As to the dropping of leaves and buds, has there been a change in amount of light or watering? As the weather cools, and if these are being grown inside, you may want to ease off the watering. Also, this is not a tropical, and should be perfectly happy in a cool greenhouse with day time temps into the low 60's to mid 70'sF, and night time temps dropping down into the 50'sF. Here in California, they are perfectly hardy to slightly below freezing, but typically stop blooming if the temps drop much below 50F. It is normal for them to drop foliage as temps drop, but they should color up first before dropping if the plant is healthy.

If you have the temps too high and the humidity too high, this could be part of your problem, and let the soil dry out between waterings. Typically Tibouchinas are not bothered by insects, but aphids, mealybug and red spider mites could be a problem when grown indoors with low humidity and lack of air movement.

Also, if you have the species Tibouchina heteromalla, the leaves normally have a fuzzy appearance that appears silvery in sunlight on mature foliage.

So all of my princess flowers leaves fell off and more are growing back, but they all have that sugary stuff, it seems to be "growing" even longer than before. When I sprayed the leaves with water and detergent the sugary looking substance mostly gets brown and burnt looking??? The leaves are curling under a bit too? How do I upload pictures on here - I don't seem to have that option? I have a MAC.

Mine is also getting this sugary substance on it.