A milky appearance on the red stripe is the classic sign of ntd.
I would detroy the fish, I had ntd years back it wiped the whole tank out, wished I destroyed all the neons when it happened.
There false ntd which is saddleback colunmaris, white to greyish fluffy appearance which goes along the back and down the sides of the fish to form a saddle shape.
My neons lasted ages with ntd it not always a fast killer.