I took my phone out of its case to clean and I found green stuff around the rear Samsung logo in the grooves. One, what is it (hopefully not mould which would be disgusting and wouldn't make sense as the phone never got wet), two, how do I clean the last of it as I've gotten most of it out and three, how did this even happen if it's kept in the same if not better conditions than all my previous phones and they've never had this