One day i got some malware on my mac. Finally removed it successfully, but this was somehow created when i first got the malware. All the forums i've found just say hit the "-" aka remove but it is grey'd out and i cannot do that.
You can not delete the Guest User.
What you are seeing is the correct behaviour for OS X; the minus button (-) is automatically disabled when the Guest User account is selected.
You appear to have disabled the Guest User account by unchecking all the options. This is enough to stop anyone logging in as guest on your Mac.
The guest user account is required by or at least in some way connected with both FileVault and Find My Mac. I’ve repeatedly encountered hearsay insisting that disabling it impairs Find My Mac, but so far as I’ve been able to find, Apple has said nothing.
In the past, I’ve suppressed it on the logon screen for users who found it confusing of irritating by setting a hidden preference. If that’s your concern, try executing the following command from the terminal:
sudo defaults write /Library/Preferences/com.apple.loginwindow showotherusers_managed -bool FALSE
Of course you ca reverse the setting with the same command, using TRUE. I’ve not tried it since El Cap, but it worked then.