Aluminium "white rust" on roadbike. How bad is my situation?
It’s not bad, it’s cosmetic only. It’s just surface corrosion that’s got under the clear coat. Do you live in area where salt is used on the roads? Perhaps the previous owner was not good about cleaning under the bottom bracket shell.
You can halt this by making sure to thoroughly clean the bike regularly and use something like WD40 in vulnerable area to displace water and provide a protective film.