Hey mate, not sure what flower it is but it definitely isn't a Trichocereus flower. The whole flower development cycle on Trichocereus is totally different to that of the plants who have these flowers. It might be an aborted flower bud from a very early stage, but with how this looks it is honestly highly unlikely. It seems to sit on the areoles, but not in a natural way that flowers emerge from Trichocereus areoles. I am really sceptical of this, and I think it might've got stuck there by accident. But yeah, if this plant should flower at one point you know for sure.
Do you have other pics of this clone? There are some cacti that flower like this, but they are other genera and this looks like Trichocereus bridgesii or something similar.