Most program icons look the same after high sierra update
Do you have League of Legends installed on your Mac ?
It seems to be the cause of the issue for many other mac players.

https://boards.na.leagueoflegends.com/en/c/bug-report/rA0p1rYx-client-updating-on-mac-high-sierra-messes-up-other-application-icons

Unfortunately there is nothing you can do until Riot acknowledged the problem and deploy a fix.