1st Layer: Coarse Gravels
These gravels served as a drainage for excessive water that’s being watered into the terrarium. Too much water stay stagnant in the soil layer will lead to root rot, which ultimately killing the plants in the jar. Thus, we would want the excess water to escape into this layer so that the soil has a chance of getting dry in mid of watering sessions. Of course, water your terrarium only once a month or when the plant looks water deprived. Under-watering the terrarium is always better than overwatering.
So what can you do now? Do not further water the terrarium, let the water within this layer evaporate onto the soil layer slowly. Let the plant slowly absorb the water off this layer. With the overwatering, you might potentially get some plants rotting away, remove them immediately if there’s any.