Yeah doesn’t come off its silver im assuming its a scratch or something it has a smooth surface like the rest of the gunIt appears to be a blemish in the finish, but it’s hard to give a definitive answer based on a picture. Have you tried anything to remove it?
Gotcha i live in a humid area when i took it out shooting when i did not really oil the gun the spots did not rust up im assuming its just the finish wearIf you really want to know if rust or not, degrease and dry. Get area bone dry. Rust bloom should show.
If it's rust, hold the spot over a teapot and steam for 10 minutes then brush off with clean nylon/brass brush or degreased 0000 steel wool to remove loose oxide. Brush, don't scrub. Repeat until all oxide has been converted to bluing. Apply non-detergent oil to area and let set for a day. Avoid using gun oil as it has rust inhibitors and may attack your new rust bluing.
Or do nothing and keep the spot oiled.
Or you can do something like How to Remove Rust from A Gun