The top panel of the unit has some white residue. What should I do?

The residue is from chemicals and minerals in the pool water. It does not really have much impact on the operation of Betta or its solar charging capability. You can take care of it when it is time to empty the collection basket inside. Spray lightly with a water hose and wipe the panel clean with a soft cloth. Do NOT use a pressure washer and do NOT scrape the panel using any tools.