Unknown "Icon?" file in Dropbox Stack
Have to recently turn on visibility for hidden files?
I see this file from time to time in my Dropbox too, however this file exists in every directory on macOS, it's just normally hidden. If you don't have hidden files turned on it's likely just an error with the Dropbox sync. See this really in-depth answer on Super User:
https://superuser.com/questions/298785/icon-file-on-os-x-desktop