That is shading implying its precense. The “white triangle” is the “lit” skin on the nose. But any countours (besides the very tip) are omitted.

Noses are difficult to draw in the anime style due to them being a pretty detailed part of the face.

As the mouth and eyes are “simplified” in comparison to the real thing, noses tend to look weird the second you add definite lines.

Different artists solve it differently, often by drawing something that merely suggests its precence.