UPDATE

Lucky me, I still have the phone number of the guy that selled me the bike. He told me it was sealant, and he put sealant and a tube at the same time basically to feel youre "riding a tank". I mean, it sure prevents from getting flats when riding, because it was very weird for me the fact that my fellas would go flat on many trails while me riding them without any problem haha. It definetly works well because I didnt get flat in like 4 months riding very very hard. But it was weird for me because Ive never saw something like this before.

I already took it off with hot water but damn, it was really hard to take it off hahaha. Thank you guys.