Cool algae!

[JackGulley]

I had noticed this funky macroalgae growing in the back of my tank for a while, but today while cleaning I decided to investigate it and put it on display. Pretty cool!

 

[WhistlingBadger]

That is cool! Looks almost like susswassertang.

 

[Seisage]

That is cool! Looks almost like susswassertang.

That's what I thought too!
In fact, it looks so much like it that I'm wondering... is it possible you got a very lucky sort of hitchhiker with a recent plant purchase?

If it is an actual alga though, I'd guess a species of Ulva, potentially

 

[JackGulley]

I just looked up pictures and I think that’s what it is! I had never heard of it before. I don’t know how long it’s been in my tank, I just noticed it inside the Java moss one day