This was used to hold some alchemical liquid or substance last year, or maybe the year before. Because it has a crack in it, I suspect it was a solid, a salt of some kind. Then over a year it has absorbed water from the atmosphere and the solution has crept into all the little cracks and pores of the pot. Finally when it reaches the outside, the water evaporates leaving behind little tufts of salt.
Either that or a really weird fungus.
I shall test it and find out, using a burning piece of wood.
Of course you should really clean out your vessels properly, but then on the other hand that would mean no penicillin or no weird things like this pot.