Just picked up a new to me cross check for commuting and riding where ever (roads, paths, dirt, gravel- general getting around). I removed the seatpost and shined a light down the tube to see if there was any rust in there, and noticed there was some rust where the edges of the tube meet the bottom bracket (on both the frame and BB). My questions are 1) Does this mean it is likely there is further rust in the frame? 2) Should I get the rust off or leave it there? 3) Is this a common thing I should not worry about?

Thanks for any help... first steel bike