Have folliculitis in pubic area, using flucloxacillin + kenacomb cream. Now have sticky white crust on undies, what is it? Is this a worrying sign?

Probably not. Although, it is really hard to know without seeing it, but if the antibiotic you are taking is the right choice one, crusting may represent healing. I am not sure what you are using the cream for but mixed in with pus it may dry up causing crusts to develop.