APOROPHYLLUM x 'SCHNEEWEISS'
Here is the aspect of the same bloom, photograph taken 5 days after the previous one. The bloom is approximately 4 cm long and the evolution appears quite modest. And for good reason because a few days after, the bloom was completely faded without having opened more than on this photograph.
Then I thought that this bloom had strange resemblances to those of Selenicereus innesii which by the way is not a Selenicereus.
Selenicereus innesii, originating from the island of Saint Vincent in the Lesser Antilles, has blooms which do not always open and which, according to the conditions of culture, are completely male, completely female, or normal from the point of view of the reproductive organs. It was proved that this plant, that one believed being a botanical species, would be an hybrid between Aporocactus flagelliformis (currently Disocactus flagelliformis) and Weberocereus.
(Sources: Wikipedia and private correspondence with Eckhard Meier)
To better understand I proceeded to a longitudinal cut of the bloom.