Hello,
what you refer to as white discolorations are probably so-called "efflorescences," which mainly occur on clinker bricks. As far as I know, both the clinker brick itself (depending on the type of clinker brick) and the joint (i.e., the mortar) can effloresce because certain minerals can be washed out, which then crystallize on the surface and form this white stuff. It is said to be especially bad with ready-mixed mortar, which can be built in for several days, as it contains so-called retarders that promote this efflorescence.
You can easily remove these crystallized minerals with diluted hydrochloric acid. Please observe the safety precautions when handling acids (eye protection, hand protection, etc.). Don't forget to rinse with water afterwards.