Its a common observation by aquarists and coral farmers.
Do consider that the various supplements that you add to your tank and know are working (iodine, etc) are in concentrations of mere drops/parts per thousand (or even more dilute).
Now consider the various potential contaminents on your hand, under your nails, on your arms, etc from daily life (work, contact, bathing, etc). These irritants can be in concentrations of parts per hundred.
The natural and unnatural matter we bring into the tank on our hands and arms are most always not harmful... just irritating to corals.