Schnirman, et al. (2017) talks of such a risk.
The mask, which touches the patient’s face, should be cleaned daily with distilled water. Rinsing the face mask or tubing with tap water, instead of using distilled water, or adding tap water in the humidifier is a potential source of legionella colonization.
Sterile distilled water cannot sustain legionella growth (Joly, et al. 1986). Therefore I would not hold much credibility to the idea that you don't need to throw the remaining water out in the morning. Fresh water is to be used each night. Adding tap water in the humidifier is a potential source of legionella colonization (Schnirman, et al. 2017)
Most recommend that the tubing and humidifier be cleaned at least weekly. Some suppliers also recommend cleaning the mask and tubing with vinegar and water daily. In addition, masks and tubing are designed to be disposable; they should not be used more than their recommended periods, which is generally 3–6 months for CPAP masks. (Schnirman, et al. 2017)
Joly, J. R., Déry, P., Gauvreau, L., Coté, L., & Trépanier, C. (1986). Legionnaires' disease caused by Legionella dumoffii in distilled water. CMAJ : Canadian Medical Association journal = journal de l'Association medicale canadienne, 135(11), 1274–1277. pmcid: PMC1491378
Schnirman, R., Nur, N., Bonitati, A., & Carino, G. (2017). A case of legionella pneumonia caused by home use of continuous positive airway pressure. SAGE open medical case reports, 5, 2050313X17744981. doi: 10.1177/2050313X17744981 pmcid: PMC5721956