Answer: The white stuff is most likely powdery mildew. It can be controlled with Neem oil. the yellowing might be caused by the powdery mildew or by a manganese deficiency. You didn't mention whether or not you're regularly feeding your palms - there are several brands of fertilizer especially formulated for palms. Apply according to label directions and you should see an improvement in the color of the fronds.
Best wishes with your robellinis!
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