Hi there, im offshore and can only see lo-res picture so can't make out where the discolouration is. If it is discolouration rather than a stain; it can be fixed through a procedure known as 'retrobrite'. I have used that on a Dreamcast before and a BBA I bought with excellent results. I used a 10% cream peroxide mixture. google 'retrobrite'.
Dreamcast: Blue Swirl, 60hz, White LED, Removable battery holder, Internal SD card reader, BBA, DreamPi, RGB Scart