I have black bits in the water dispensing from my fridge - amyone know what it is and how I can get rid of it?

The black bits are not solid - more of a gel or slime like substance! The fridge is badged Smeg but I think it's manuafactured by Whirlpool. The water dispenser is plumbed and so fills automatically from the water supply. I don't have this problem with tap water so believe it's to do with the appliance. Would be great if any one has any ideas on how to fix this.