Electrify is an artist's name, and while it can also be a verb, it's not the tense we use for general tags. I am unable to change it to an artist tag for some reason.

Wait, just checked the source... Watermark says ELectrify but their Furaffinity username is EIectrify. His second account has an L, not an I though.

That's because it is an artist tag, or rather the system thinks it is. It's a mildly confusing glitch that happens from time to time, but it's easy to fix. All you need to do is change it to a different type (like copyright) and then change again to artist and it should move for you.