Help! My 3 day old terrarium already has mold :(

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I don't see any mold in your photos. The remaining plants look very healthy. How much mold was there? Are we talking a spot the size of a match head? A dime?

White mold in enclosed spaces is usually a saporophyte (that is, it grows only on dead material.) It will not invade your living plants. If you remove the dead material the mold has nothing on which to feed. I ignore white mold on the few mounts I have inside jars.

I think there's a real possibility you have no problem at all. You certainly do need some air circulation, but a little mold on a dead leaf from time to time is nothing to worry about.

The Dyackia really does need very high humidity, by the way. I would keep it near the bottom of the terrarium.