It's likely a build-up of salts (from your water and/or your fertilizer) left on the outside of the container. Clay pots 'breathe' through micropores, and the salts are left behind as solids when the H20 evaporates.

A stiff or wire brush may help. If these are empty containers, you can soak them in a strong vinegar solution before taking a brush to them.

Rinse/scrub off as much as you can before you reuse the pots, especially inside the pot - if you stack them, what's on the outside will get to the inside of the lower pot. Too much salts, from fertilizer or anything, will burn plant roots. Try to cut down on your rate of fertilizing next summer - you are overdoing it a bit, it seems. And do water really strongly every so often, so the water runs freely out the bottoms of the pots, so you don't get a salt build-up on the top of the soil as the fertilizer(s) you used dissolved.

im fertilizing with neptune's 1 time every 1-2 weeks. i have a feeling it's what's in the tapwater, because it's happening with somene else, on a different fertilizer regime (much less than I do)

That is called "efflorescence" and is the dry deposits of calcium from the pots soil, or maybe even what the terra cotta pot is made from, as moisture moves from inside the pot to the outside, evaporation. Some people believe this adds character to their pots and leave it and many ancient pots have a lot of it and still grow plants quite well.
Because the deposits are calcium (mostly) and are alkaline a wash of a vinegar (acid) solution will remove it best.

Here is a link that might be useful: Efflorescence

Just water hardness plating out there. I kind of like it on my teracotta pots- gives them a neat "old world aged" look, IMO. Sometimes it bunches up nasty, though.