Calanthe trouble

[Longroots]

I bought 3 Cypripedium and a Calanthe about a week ago. The Cypripedium seem pretty happy, and have grown a little since slip potting them into the substrate. They're in a shaded area that gets some dappled light that is shielded from the wind too.

The substrate is about 40:60 LECA to fired clay (what i use for my bonsai). Since simply slip potting the 4 pots, the calanthe developed this kind of dark green mottling and seems to be declining. The substrate it came in was a mix of soil, coir and tiny bits of bark (all fairly broken down).

I'm assuming there's no saving this Calanthe now, but for the future, why did it react so badly and quickly?