If the old leaves from your Droserae are "composting" there are some real problems associated with this: compost forms as an action of nitrobacter: a microscopic bacteria that breaks down organic compounds and releases nitrogen. Ideally, this should be entirely absent from your substrate, as it is absent from the habitats where these plants have evolved. You do not want nitrogen or other nutrients to be added to the substrate, just the opposite(!
, and old leaves should not break down at all if the bacteria isn't present. Old leaves present other problems: they are food for various fungi that will grow on them. The humid conditions of terraria are perfect to grow molds and fungi, and these in turn can attack healthy leaves, and even at times spread like wildfire, destroying the plants and moss in the terraria in a matter of days. I recommend that you remain very attentive to this, and remove your dead leaves. Outside in habitat other factors are at play which discourages fungi, but in terraria life is much different.
Growing Droserae well means maximizing all conditions as much as possible. Even though the individual details may not seem like much, it's these small things that result in healthy robust plants. Providing good water, the correct day/night temperature, nutrient free substrate, adequate light, ample space, good air circulation, and changing the water in the trays frequently, flushing the pots from above on occasion all contribute to good growing.