Do you have hard water where you live? Hard water is water that contains a lot of minerals, among others calcium, and this can lead to a problem in your home. Calcium will be deposited on your faucets and showerheads and can build up quite quickly depending on how high the content in your water is. You can’t really prevent this from happening, but you can take care of the buildup by using vinegar.
Vinegar softens the calcium to the point where it will come off easily. Usually, the easiest way to do it is to soak a towel in vinegar and wrapping it around the faucet that needs treatment. For a showerhead, you might need to get a bucket tied around it that you put vinegar in. It should be enough to let it soak for a few minutes. You can try it out first and see if it comes off easily. If not, let it sit for a while longer. Using too much force could damage your faucet, so you need to make sure it is softened sufficiently.
If there is a lot of buildup, you might need to work it off one layer at a time.