Humidifier Residue Question

I just purchased a used (only 7 hours run time indicated on machine) ResMed S9 with H5i humidifier. When I brought the machine home I gave it a thorough cleaning and wipedown with sanitizing wipes. I took the water chamber apart and washed it out with a little dish soap and when I was drying it there seemed to be a texture to the inside of the aluminum water pan. I thought it was how it was designed and let it air dry. I ended up filling it with distilled water last night and used it for one night. This evening when I went to empty out last nights water and refill I realized if I rubbed enough the residue came off.

I ended up scrubbing it out with dish soap and a scotch brite pad. it is now clean but has anyone experienced that or have any idea what kind of residue could have been in there with such little use? Makes me worried what I breathed in for the few hours I used it last night.

Purchase a new water chamber and use only distilled water.

Mix a cup of water with a cup of white vinegar.
Microwave until boiling, then pour into the chamber bottom.
Let rest until room temperature.
Wash and rinse again.
If it is gone, then it was calcium from using tap water. Good as new!