@Altabosch, like Rashid510 indicated, it is the butyl rubber that is necessary to create a seal between the black bottom cover and the lower battery casting. I would not recommend trying to "clean it off" or remove the oozing black butyl. You don't want to compromise the seal that is protecting your internal battery from moisture and dirt. It might be a bit unslightly to see the black ooze, but it is doing its job to keep your battery protected.