Crux is quite a different beast to Datomic, in that it is schema-less, designed to work with Kafka, bi-temporal and some other architectural differences. Similarities are that they are both built (mainly) in Clojure and support Datalog queries (albeit different dialects)
Disclosure: I work at JUXT but not directly on Crux
Disclosure: I work at JUXT but not directly on Crux