Is it a residue, or just discolouration of the seal?
If it's not a residue, then it might be that the seal has just discoloured at the point of pressure, particularly if it makes contact with stainless steel.
If it has discoloured, I really wouldn't worry about it as long as its not leaking.
If it is residue, then run an empty wash on hot with some washing machine cleaner added - Aldi sell one (apparently there's no horse meat in it ;-). A purpose made cleaner will descale and loosed hardened powder deposits, and get rid of any smells present.
There's probably a filter on the pump, if there is, trying cleaning that out. The handbook will show how if it's serviceable by the user.
If you want to check your water, tie a piece of white clean cloth over the tap spout, and let it run for several minutes, if there is anything untoward coming through, it will show when you remove the cloth.