Coraline algae

If I remember my chemistry right, the hard water (being more acidic) strips away much of the moisture in your skin. When you wash your hands with harder more acidic water, the soap (being very base) "bonds" (for the lack of a better term) to your dried skin cells to try and balance the lowered pH level of your skin. That's what gives you that slimey feeling that won't go away. Basic compounds naturally have a slimey feeling, where as acids are more "dry" and harsh.