Double mastectomy with free nipple grafts in 2014. Ever since had a white lump under one of the grafts. What is it? (Photos)

It doesn't hurt, doesn't itch, isn't inflamed or hot, but I'm still worried about it and would like to know what it is. I want to be sure this isn't an infection or something that needs prompt treatment, especially because I've ignored it for about two years. It has not changed at all/gotten worse throughout the years, which I think is a good sign. Thanks in advance everyone!