Yes, it is normal. The CO2 from the air binds with the limewater at the tip to form calcium carbonate. I just use a flexible hose to dispense my limewater and squeeze the end every few weeks to break off the build-up. That works fine. I keep mine above the water level. I think I read that if you submerge it, it gets clogged worse.
You can also dip the tubing is vinegar for a few minutes, as this will dissolve the clog. Or you could run vinegar through the tubing to get all the clogs out (not into the aquarium, but in a separate container...then just rinse with water and re-connect to kalkwasser system.