Whats the white forming on bark?

[opaline]

I agree with Amey its calcium/ magnesium deposits. The wood bark is porous and textured and will act as a filter to collect deposits. Fairly common with use of tap water but can also be the fertilizers and or hormones? you use in misting. Otherwise have a look at the media your using in pot and assess whether the media has decomposed in pot under surface. This might be worth considering to use as a process of elimination. If the media is rotting it could be a type of fungus but sway more to mineral deposits. Try a fungicide as part of your assessment.? maybe

[orcoholic]

I noticed the same thing on my orchids when I started using MSU fertilizer. I don't think it's harmful as orchids seem to be fine. I stopped using MSU because it became too expensive to ship in (I use the 25lb bags) and began using Jack's RO Special, which i can pick up at Jack's. The deposits disappeared and stopped forming.

One thing you may want to do is check the Ph. When I was using MSU, I wasn't paying too much attention to it. With Jack's I am. Ph should be around 6.5 to give orchids the best chance to use all the nutrients. Thus, the deposits may have formed because my Ph was out of whack.