Golden ball cactus question?

I have a golden ball cactus. I did something to it that made it go brown near the top. It was a pretty big brown mark and I'm pretty sure that that part is dead. It's about 1 1/2 inches long and 3/4 inches tall. It is also near the top of the cactus. Now, at the top, there are all of this white fluffy stuff and it could be a flower. Will this mark affect how the cactus grows andis the cactus dead even though most of it is green with the small yellow spike things. Just in case, I got this at the Desert Botanical Gardens in Phoenix and live in the Western suberbs of Chicago.