Calcium typically forms and becomes visible on swimming pool tiles at some stage in the life of almost every pool.  It is a very common issue that pool owners face for various reasons. Calcium forms on tiles when they get wet.  The water evaporates and the calcium and minerals in the water are left behind to dry and solidify on the tiles. calcium formation will almost always happen on waterline tiles (and water feature tiles) due to heat, wind and fast evaporation, particularly in summer.