Thank you for your questions.

The white substance is powdery mildew. It is the result of high humidity around the plant. Increase air circulation and make sure that the plant is not sitting in any water. Remove any affected leaves. Here is some information from Penn State Extension: https://extension.psu.edu/preventing-diagnosing-and-correcting-common-houseplant-problems .

As for the yellowing leaves,they could be the result of powdery milldew on them.  However, it looks like this was a recently purchased plant.  It is most likely that these older leaves were used to growing in a higher light environment than where the plant is growing now.  Older leaves have a harder time adjusting to different light levels.  As a result, the plant is shedding off these leaves.  

No concerns about root rot.

Regards,

Ed