Chlorine in tap water is in the form of free chlorine or chloramines. Neither of these are found in nature, and both are used to disinfect tap water.
The Cl in Maxicrop must be in the form of a chloride. Common table salt is sodium chloride. Maxicrop is derived from kelp, which comes from the ocean, and sea-water has chlorides. Plant's actually require chlorine as a trace element.