Answer: What you describe is just the natural reaction of the clay in terra cotta pots to the minerals in the water that you use. Terra cotta pots wick moisture out of the potting soil to the surface of the pot where it evaporates. What's left is a concentration of the minerals in the water. It won't hurt your plants or the pot. Many gardeners like to see such discoloration on their terra cotta pots - it gives them character. If you find it unattractive you can scrub it off with a brush.
Best wishes with your plants!
Q&A Library Searching Tips