that could be one of three things, hard to tell without knowing the plants history....ph issues...root rot...or broad mites...pray it's not root mites....which i kind of doubt, they don't just happen spontaneously....and you'd surely have more than one plant effected by the time it became apparent. could be ph issues, maybe that plant got a little more or less lime than the others, or w/e....could be the beginning of root rot....either way i'd do the same thing....make up a couple of gallons of carefully ph'ed water, with a good dose of h202 in it, and do a light flush of that plant....that ought to help reset the ph in the pot, and will kill anything trying to eat your roots....