it is salt from the fertilizer burning the plant. terracotta is very pores it quickly pulls water from the potting mix along with fertilizer as the water evaporates the fertilizer is deposited on the lip of the pot. If you look at your pot I think you will find a whitish powder starting to build up. that is why most people plant in plastic pots.

I've only fed them once but you are absolutely right as there is white on the pot. You really do learn something new every day. Is there a solution or do I need to repot into plastic?

You can cover the rims with a strip of aluminum foil. This was a common practice among AV growers before plastic pots became widely available & mostly replaced clay.

Terri