Silvery/whitish dust?
 

I've seen this for a few weeks on one of my Dendrobiums on the leaves. I've been spraying with cinnamon extract/water/soap mixture thinking it was mites, but I'm wondering if that assumption is wrong. What do you think?
http://img.tapatalk.com/186bb06f-c43f-1f1e.jpg
Since I've been spraying, it does seem to be going away though.

It's difficult to tell from the pic, but it could just be hard water deposits.

Is it a powdery deposit, or is it more like a thin layer that has separated from the leaf surface (which is what it looks like to me)?

If it's the latter, it's definitely a mite issue.

It feels powdery as it seems to come off on my fingers, but it's kinda scaly looking. I'll see if I can't get some better pics tonight.

mealybugs leftovers?

The part I've circled in red looks to be more "flaky" than that in blue, and that's what led me to guess mites - especially two-spotted spider mites that spin no webs. It could be that the blue area is just not as extensively damaged.

Ok, does this pic help. It's the top.
http://img.tapatalk.com/aeff0103-6914-a4d1.jpg
It's scaly/bumpy.

Here's the underside which is why I suspected mites. It's powdery.
http://img.tapatalk.com/186bb06f-6952-14cc.jpg

Don't know if this counts, but a 10x loupe didn't show anything other than powdery appearance....

When you rub the leaf between your fingers and draw them across the leaf surface, does it leave a trail of bug guts or any discoloration? Or take some kleenex or tissue paper and do that. Is there any discoloration on the tissue paper?

Powdery mildew
 

I know I'm a bit of a newbie, but it looks a bit like powdery mildew. If the *Rubbing the leaf* test doesn't show any discolouration, it could be that. If so, it's harmless, just ugly! Wipe it off with damp tissue!