Slug with Forked Tail

[TingeOfGinge]

I was looking at my tank today, and noticed a slug-looking thing with a forked tail.

It's pretty tiny, but noticeable to the naked eye up close. Looks to be a light brown color. It moves around at a decent pace and similarly to a snail's movement. I couldn't see a shell of any kind on it. This was the largest one, but with a closer look, there's tons of much smaller ones all over the glass. Any idea what it could be and if it's beneficial, evil, or neutral?
I tried doing a search online, but I don't think I was getting the right keywords as I couldn't find anything even close.

 

[EnterName]

Flatworm, most likely Acoel: see page 5, post #85 in my microscopy thread.

 

[TingeOfGinge]

Wow, crazy fast response! Thanks! So it looks like these guys aren't harmful based off your post. I'll keep an eye on them in case they become too prevalent though. I have noticed a lot fewer copepods on the glass lately...

 

[EnterName]

Wow, crazy fast response! Thanks!

So it looks like these guys aren't harmful based off your post. I'll keep an eye on them in case they become too prevalent though. I have noticed a lot fewer copepods on the glass lately...

There are quite a few harmful flatworms you don't want in your tank, but these Acoels are most likely just a "cosmetic" issue, aside from their appetite for copepods and other beneficial organisms like ostracods.

As long as they stay away from coral tissue (LPS skeleton is fine) and nothing in your tank seems to be having trouble, you should be fine.

There are chemical treatments that can be applied within the display tank, but they aren't 100% risk free. Some flatworms produce toxins and release them when dying during treatment, which is why treating a small population is the safest approach. Keep in mind these treatments won't get all of them. Eggs won't be affected, and some specimens simply seem to tolerate reef-safe treatment levels well enough to survive. A wrasse or certain other fish can be a natural way to keep their numbers down. You can also siphon these guys out manually with a thin air hose so you don't lose much water in the process.