Found this on the leaf of my Anubias. What on earth is it

[Clockwork-crow]

It was waving that thin appendage around on my finger and in the air. I was. Creeped out by it to say the least. 

[Colu]

It looks like a leech

[Clockwork-crow]

Should I be concerned by the fact it made its way into my aquarium? I haven't added anything, plants or livestock, into it for a couple of months, and all fish are accounted for

[Colu]

They usely come in on plants I would just remove any you see 

[NanoNano]

As far as I'm aware,  there is no such thing as a "herbivore" leech- they all eat flesh and blood.  The aquatic leeches that I've seen have all had their head at the smallest part of their body (the part pointing towards the floor in your picture) and have a a very distinct "inchworm" type movement where they double their body up and then thrust their head forward.

I'm with @Colu - You want to remove any of these that you find and dispatch them so they don't end up in your local water.  This guy is is either feeding on or planning to feed on your fish and snails.  it might not out right kill any of them,  but it can introduce pathogens and/or cause weakness and anemia like issues via it's feeding. 

[HeatherTuller]

I kept getting them in my tanks with live black worms. Even after I tried to separate them.  So I stopped ordering them. But I was left with leeches afterwards. They loved it when I’d feed the tank white worms. I’d watch them eat them. But trying to catch them wasn’t easy. I bought some cheap glass trap on Amazon. That has helped. Also, my pea puffers seemed to take care of them on their own. lol, so I didn’t need to worry about that tank, but I wouldn’t consider them to be as good as the glass trap. It took them awhile/don’t always get along with other fish. I have gotten them from other things, such as plants, but it wasn’t nearly as many as the black worm fiasco. Probably baby leaches that I couldn’t see?