Name of Invertebrate?

[mst3kguy07]

I have a very small, almost microscopic invertebrate in my tank. I can't recall the name...acoen/acaen?? It reminds me of an amoeba..but obviously larger in size. It's opaque and usually comes out at night feeding on the algae on the back wall and glass of my nanocube. It's "tail" end is forked. Can anyone help me with the name & any links with photos. I'm wanting to learn more about it. All I know is that I was told it was a "good thing" to have in my tank....not sure if it's beneficial for the tank environment (ie. algae eater) vs. food for my other fish, crabs & snails.

[ptychoptera]

Acoel flatworms. Various sources mention it being a species from the genus Amphiscolops.