The yellow on the one leaf looks like sunburn. Nothing you can do now, so don't worry about it.
Epiphytes do much of their respiratory gas exchange through their roots, rather than leaves, so - as has been said above - it is important that they have lots of air flow around them. Soil, or even too fine of an orchid potting medium, will trap water in the spaces between the particles, cutting off that air flow and suffocating the roots.
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