Looks like a type of maidenhair fern, Adiantum species, but photo not clear or detailed enough to determine exactly what type. These prefer to be grown under cool conditions in consistently moist, pretty organic soil and with high humidity. Indoors is often far too dry for them unless you take specific measures to raise the humidity level.

I'd repot in something far more moisture retentive than cactus soil and investigate ways to bump up the humidity in its growing location.