Computer security is never 100% perfect, so you should also be a little "worried" about it
Before you continue, disconnect your computer from the network/internet to make sure that nobody can access it from the internet (just as a precaution). Then, try to find if that specific user '-' owns some files/directories, look at logfiles, run "rootkithunter", ...
The username '-' isn't very common, so I doubt if it is created by a software package that was installed.
Maybe the user was created by mistake (ie a "useradd" command with incorrect syntax may have led to the creation of the user) or maybe the user was created by someone else who has access to your computer (not necessarily with bad intentions).
You could also:
1. Disable the account to see what happens (ie block logins).
2. Rename the username '-' to something else.
3. Delete the account altogether using 'userdel'.