that is black rot
can be from overwatering during the cold or slow growth times, or other damage to tissue
as its on the tip, and side, it may already be systemic. Is it rooted? new arrivals??
it might heal, and just leave a scar, or the whole thing might turn mushy soon(death)
if it was a larger plant i'd say, cut off all the rot, (keep cutting until you hit white tissue with no rot) then dust cuts with fungicide(sulphur)
as it is, i'd leave it be and hope for the best
some tips, keep it dry, dont water it much-if at all until healed, leave in a mostly shady dry outdoor location
edit: i looked at the pics again, they are likely new arrivals, now being grown indoors, they may not recover with inside conditions