Probable Circuit Board Corrosion

The control button cover. Note that the perimeter “dirt” is in straight lines, and even bends at the corners to match the elevated part of the cover. This implies  that  we  are  seeing  that  the  edge is  more  adhesive  than  the  outer portions. IMO, this is not insect debris.

The KingBright LED  shows thick corrosion buildup. It appears the LED is leaking.

The battery is a big give-away. Note that both sides show rust or corrosion, although the battery has not leaked, as there is no blackened area around the perimeter. Additionally, the casing for the battery shows corrosion near it. The battery compartment apparently holds the moisture for long periods of time.

The other areas, such as this, look to be corrosion as well. Maybe moisture has been able to enter a few of the components.

Ideas:

Call the manufacturer?

Use silica gel?  https://resources.altium.com/p/circuit-design-tips-pcb-moisture-protection-humid-environments