Answer: The pink plant is possibly a hypoestes, or pink polka dot plant. If the terrarium has a lid on it, and is therefore a closed system, you should not need to do anything to feed or water it as long as there continues to be a slight bit of condensation on the glass. If there is dripping water inside, it is too wet and should be allowed to air out a bit to avoid mold problems. If there is never any condensation then it may need a small amount of water, start with a mere sprinkling and see if that is enough to keep the soil barely damp.
If there is no lid, then you could either add one and treat as above or else water as needed to keep the soil just damp, not sopping wet and never dry. How often will depend on the ambient temperature and humidity in the room as well as the amount of air circulation.
In most cases it is better not to mist African violet foliage but water carefully at the root zone. Eventually most plants do either outgrow a terrarium or else become mature and in need of rejuvenation, and so need to be divided or pruned periodically. The soil fertility and amount of light will determine how fast they grow.
These plants will do best in bright but indirect light. Be sure that direct sunlight does not cause the container to overheat if it is closed with a lid. Have fun with your terrarium!
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