riot.im [1] (which is based on matrix.org) seems a good, decentralized, open messaging app. They have relatively nice mobile apps and they promise to soon release end-to-end encryption based on the OLM [2] ratchet which is similar to the Signal encryption. In contrast to Tox, Matrix relies on federated servers. Tox is pure P2P which, in my experience, never works very well on mobile devices.
[1] https://riot.im/ [2] https://matrix.org/docs/spec/olm.html