So normal! Brick discolouration is usually caused by a lil thing called vanadium staining but it generally fades with time.
It’s caused by the vanadium salts found in the clay used to produce bricks of a lighter colour. The salts are left on the surface as excess water migrates through the brick. The ‘discolouration’ is totally natural and doesn’t mean anything is wrong with your bricks. Phew!