cloth pot residue

i started using these 10" cloth pots felt i think and everything seems fine accept i noticed a yellowish brown residue on the outside of the pots what do you think it is the plants are doing well so if this could be a problem i would like to nip it in the bud as soon as possible thanks in advance

water seepage?

when you say cloth... i am envisioning the brown cardboard pots... is this correct?

picture please

these pots are actually fabric they are called smart pots by high caliper smart growing system i think they are felt at least thats what they feel like
the batteries in my camera are dead but i will put up some pics as soon as they are charged thanks again