It is adware. Probably from downloading Flash from a site other than Adobe. Always be sure to check updates and update fish using the settings in System Preferences > Flash Player
Force Quit Safari (command + option + esc). Then, open Safariholding the Shift key.
If you still have problems, Empty Caches. You can either choose Safari menu > Clear History (Or, if you do not want to remove History go to Safari menu > Preferences > Advanced and check the “Show Develop Menu in Menubar”. Then, on that menu > “Empty Caches”.
If the problem persists download and run MalwareBytes. Malwarebytes was developed by one of our own colleagues here in ASC. It gets rave reviews and is about the most proven anti-malware software for Mac.