Received this several times and had a hard time getting rid of it. (Notice "Your phone has (13)serious virus" - its grammar and spacing. View attachment 234674
Don't click on it just clear it. My wife's S5 got it once other sites not porn . Just X out of it and clear your cache. If you still get do an Factory reset.
If you have any apps from Cheetah Mobile installed, uninstall them. What you're seeing is typical malware from CM.
Unfortunately samsung now includes cheatah mobile ram management and file cleaning in the so called "smart manager" package. However you can disable them using package disabler.