You need to inspect it closely and see if there are any signs of irritation, redness, inflammation, sores, or infection, and if there is have your bun seen by your vet. If it is just healthy pink skin, this can be normal for rabbits to have a bit of pink skin showing on their heals. They may even develop a callous, which is ok as it protects the skin from rubbing. The important thing is to keep an eye on it and make sure it doesn't get sore. If you aren't sure or in doubt, best to have your bun checked by a good rabbit vet. Just from the little I can see in the pics, they don't appear sore to me.
You can do some things to help reduce any rubbing that might be occurring to the hocks. If the carpet you are using has a rough surface, change to a softer carpet(if it's removable), or you may need to cover the carpet or remove altogether. Some things that can help are soft fleece fabric, soft padded blankets, or I like the foam tiles with the wood veneer surfaces. Replacing the flooring with a hard flooring like lino, can sometimes even help as it reduces the friction occurring. But hard surfaces can also sometimes make it worse, so it's just something you would just have to try, and keep a close eye on the hocks to see if it helps or not. Also some rabbits hate hard floors and won't walk on them.
Wire cage floors can also cause rubbing on the hocks. A better alternative may be to have solid flooring covered with something soft like fleece or hay if your bun won't pee on it, or foam tiles. For the litter box I would take the wire grate out and just use the litter covered in a fresh layer of hay, making sure to clean it regularly.
Finding flooring that works best really can be trial and error. I tried fleece for one bun, but the type I was using(there are different types and some are softer than others) really didn't help. So now I'm trying foam tiles, which can work well if your bun isn't a chewer(don't use if your bun will chew and possibly ingest). The ones that I like best have a wood veneer on the top, making the surface smoother, which creates less friction for rubbing fur off and also makes sweeping up easier. Some of the other types can actually have a bit of a rough surface and may make sore hocks worse.
Here's some info on sore hocks.
http://www.medirabbit.com/EN/Skin_diseases/Mechanical/Pod/Podo.htm