Possibly a Pteris fern. Best not to grow them in with CP, they have numerous black fibrous roots which will quickly take over a container. Your N. ampullaria will suffer, so best to transplant the fern to another container if you wish to keep it.
To me, it looks like Lygodium microphyllum, or climbing fern. It originated in Asia, and in some parts of the United States (especially Florida), it is considered an invasive! As the plant grows, its leaves look quite different than what you currently show -- they become more finely divided and are quite attractive.