ok, ive seen these in a lot of pictures and wondered what the heck they are? They are a little sleeve that is on the outside of the mast, and is usually a different color. And its always under the boom/about 4 inches from the gooseneck on the mast. Here is a picture that i found.
various things:
At many regattas the fleets are split and the organizers give us various bands to put on the mast. ( red / blue / green / orange are typical four fleet split tools)
The sailor may have a spray top folded and banded to the mast.
Could be a water bottle
could be his shaving kit so he can "dude up" before meeting his fanclub of 100 babes on the shore