What do you think it is?
Frankly, Cap'n, I think it is totally unnecessary to run anti-virus on a Mac.
Every Mac OS X for the last five years has had its own built in xProtect anti-malware built in that will identify all the known Mac malware already and any other commercial anti-virus unethically TURNS THAT OFF so they can claim they found malware AFTER it downloads! Apple's xProtect prevents it from downloading, being installed, or being run in the first place, warning the user of it's presence. Your Mac contacts Apple every day and checks for any new malware definitions to add to its xProtect files. Ergo, there is no need for a third party anti-virus to be purchased or installed.
Essentially 99.9% of any malware that ESET, or any Mac anti-virus, anti-malware software is going to find is malware for Windows imbedded in email that cannot effect your Mac in the least. The other 0.1% are OS X trojans that you might inadvertently download, if you were to attempt to download something from an untrusted source other than the Apple Mac App store. . . all of which Apple's xProtect would prevent from happening. IF, you should by some means, copy it onto your Mac, xProtect will tell you:
In answer to your question as to why it may not be working. These commercial anti-viruses modify the OS to hook themselves into the OS at a very basic level. . . and switch off xProtection. Yosemite is a re-write of the Operating System and ESET's hooks just may not be in the same locations anymore. Alternately, the run at start-up may have been disabled and fixing it may be as simple as requiring a re-install of the ESET 2013 to re-establish those files.
As I said, I generally disagree with the philosophy of running anti-virus on any of the Macs I am responsible for. . . and that is quite a few, as I run many networked Macs for several businesses. Not once in over 16 years has any of the OS X Macs been infected by malware. . . and in dozens of Macs, there is not one instance of third party anti-virus software. Only Apple's xProtect.
I once even ran my personal OS X Mac without a firewall for two years with no anti-virus to see what would happen. What happened was NOTHING. There are websites that can test your exposure and when I had that Mac tested, they did not even see it was on their websites. It was invisible to them.
I would not run a Mac today without a firewall, but then it was safe to do.