The photo is very small. I cannot see exactly what is going on. However, I do believe I see some white residue on the stem. This can be an infestation or mold.

Since I cannot see the whole plant and how it is growing, my guess is that you can cut off the yellow part of the stem. The other shoots are in a position to not be affected by this removal? It looks like they are on a different stem or at least not on the part that has yellowed.

If the whiteish residue is removed from cutting off this part of the stem, it may just be mold from the rotting plant. Otherwise, you will have to deal with that as well.