The deposit is sea water calcium carbonates
And, in the head discharge line, may also include urine crystals.The cure AND prevention: a cupful of undiluted distilled white vinegar flushed all the way through the system once a week. It'll take a little time to dissolve the existing buildup, but it will and will prevent it in the future.Use ONLY distilled white vinegar...it's more acidic than cider--which means it's more effective...and it's distilled from alcohol, and therefore leaves no residue...whereas cider and wine vinegars are distilled fruit juices that leave a sticky residue that traps waste and sea water minerals.Be sure to flush it ALL the way through to the holding tank...do not leave vinegar sitting in the bowl. You might want to check out the link below for more detailed info on this subject and just about everything else you need to know to maintain your sanitation system.