+1 on Malwarebytes. It is the only non apple app you will ever hear apple suggest.
You also want to make sure your homepage in all your web browsers isn't hyjacked to something like chum search. Remove your extensions that you don't recognize. Go into system preferences and remove any profiles that you don't recognize. These are under "profiles" not users and groups. If you don't see profiles then none are installed. These are 3 things that malwarebytes won't fix.
My advice because I deal with this daily. Your anti-malware on your Mac is you. You won't get malware on it when your MacOS is fully up to date currently (10.12.4 or 5 cant remember) and you don't help something to install. If you don't click to allow something to install it won't happen and you won't have malware. Stop downloading crap a website tells you to and installing it. Its that simple. No fancy programs to slow down your computer. You and your little clicky fingers to keep things up to date and not install stuff that is questionable.
Don't click on the "you must install flash player to view this website" message when on a website. Don't click on the warning that comes up when you are on a website that your computer is infected, do not call any number that comes up on your screen. (this website has had this issue too) If you need a number for a company. Don't ask google...google has ugly friends and they lie. Go to the website of who you are contacting and view the contact info. Many companies don't have numbers at all. Why do people call fake apple numbers and buy gift cards to give to the scammers is beyond me but it happens to people that say they won't let it happen to them.
A Mac will not tell you there is anything wrong with your computer such as viruses or malware. It won't tell you its infected or your iCloud account is about to explode if you don't call some number and give them $1000 in iTunes gift cards. Same with your iOS devices.
It will quietly tell you you have low disk space. Thats all it says. It may take you to storage in about this Mac but it isn't going to tell you to click here to fix it. There is no application. The current MacOS is very good at managing itself and your computer. it doesn't need help.
How to free up space on your Mac. Direct from the apple website. No fancy things required
https://support.apple.com/en-us/HT206996
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