salts from your water.. accumulating on the top ... and eventually the cup itself ... [and salts includes the ferts you might be adding .]

they are probably big enough.. to go up a pot ...

so change out the media ... and make sure to use a media... that allows more thorough water drainage ... that is part of the problem with salt build up ... improper flushing ... there are no large structures in your near all peat media ....

trees in general.. like a sip of water.. and then the rest.. just about all flowing out the bottom ...

just going with filtered water.. only addresses some of those issues ...

these are general 'tree in pot' theories.. not specific to citrus ....

ken

You are probably keeping your soil too wet, maybe watering too often. Al

ken is correct