Colloidal Silver is unstable, and would likely react with the metals surface of an ultrasonic humidifier.
With that said, it is likely that particles left at the bottom of a reservoir could be due to Colloidal Silver either; dropping-out or plating and precipitating over time with use.
Though I wouldn't worry about such things either way, as this would not likely affect the effectiveness of the CS being breathed-in - assuming that the CS is in fact renewed with every use.