You need to post a photo...maybe yo tried and didn't work? Try again in a comment.

Can't see discolored parts, close-up and clear photo would be helpful.

Many cacti will have some corking with age which is normal. I don't know if that is the case with your plant. I am not sure about needles falling off so easily.

You say it is growing at an alarming rate at the top . That may be due to low light conditions. It shows very often in etiolaed growth - stretching for more light.

It’s fine just needs more light as Rina has stated. Fast growth is not good growth for succulent. They need lots of sun to grow strong and compact or you will end up with a wobbly flimsy cactus like this.

pics from Google.

This one was receiving low light for a period of time. Then started getting higher levels of light. So now it looks funny.

If you‘re worried unpot and check the roots, but your Mammillaria looks fine from what I can see. The new spines are fresh that’s why they’re white and clean.

If it’s been a couple yrs since you’ve potted it up. It could be time to give fresh mix:).