I'm wondering the same issue. Maybe just a paint crack or maybe some more serious. Broke bike polo fork last summer while 180 nose endo turns and in the beginning crack were exactly the same on ebay Eddy. Also color is not my favourite.
Amazing to think this bike had the clearance to take a 2.5inch tyre way back in 1990 !! Love big rubber on a Yo.
You can never be 100% sure ( no harm in asking Pros Closet to confirm) but i think its only light surface rust there, no endo damage or terminal rust. Its normal to get some light surface rust spidering underneath the paint on frames of this age.