Battery protection, gasket???

[Przemek]

Can someone, please, write me what happened with... on photos? what are recommendations?

 

[trailaddict]

Corrosion. The connectors have become wet / moisty for an extended period of time.
Try to clean it with a rag and some Alcohol.
Next time your bike becomes wet remove the battery.
You can use some dielectric grease or something else to protect the connectors.