David Hunt maintains that any Epiphyllum sensu stricto with multiangled stems rather than just flattened two ribbed stems is a hybrid. How true this is I am uncertain, but does apply to most. Yours obviously has multiangled stems at the base therefore looks to me like some form of "Orchid Cactus" commonly called "Epiphyllum Hybrids", though there is seldom any true Epiphyllum blood in them, being mainly of Disocactus or Heliocereus parentage originally.
Whether there is any Selenicereus blood in it I do not know, unless the seller made the cross themselves? Most Heliocereus have red flowers, but there is a white flowered one which may be confused with a Selenicereus and have been one of it's parents. Heliocereus amecamensis is often considered just a white flowered variety or subspecies of H. speciosus, but does not produce any flattened stems like yours:-
http://mattslandscape.com/detail/?plant ... mecamensis
