When your bike was assembled, grease was used to coat the post and/or the inside of the seat tube to help prevent the seat post from getting stuck due to galvanic corrosion. The greenish tint at the top of the collar is the grease staining the paint. A cleaner wax like this Amazon.com: Meguiar's A1216 Cleaner Wax - Liquid - 16 oz.: Automotive should remove the stain, and also clean and protect the rest of the frame.