Corroded faucet aerator:

A faucet aerator is a small, round device that you can screw onto the tip of your faucet to create a more consistent, splash-free stream of water. Aerators usually create a mixture of water and air, making the stream smoother.

Due to hard water in many circumstances, they build up corrosion. It is suggested that your

using a small zip-lock plastic bag, fill it with vinegar, just enough to cover the aerator, secure it with a rubber band or tie of some sort and let it stand overnight, more often than not, the hard-water corrosion will be gone.