Don't panic! You're probably seeing kahm yeast which looks like a layer of white scrum. It's not dangerous! Simply remove it with a spoon and seal the lid again. (If you're really worried and think it's necessary, strain the juice, wash the berries, and combine them again in your jar. Then continue the fermentation process until the juice is done.) You can read more about kahm yeast here. If you're worried that what you're seeing is mold, here is a great article on what to do.