I got shipped a batch of subwassertang a while back and a lot of it was brown (it's similar to pelia I think) and some of the brown parts recovered after a few months, 2-3, some never recovered but had some really nice green growth. I don't use ferts or anything either, and I have a really low dinky sort of light, so it should bounce back if it's new. If it isn't, do you happen to dose with flourish Excel?
If it started to green up later it's probably fine.
Yes, it does grow similar to java moss, and shrimp like it just like java moss.
I actually have both in my shrimp tank and my shrimp are on the pelia more than the java moss
It's likely a mix of them, to be honest. The issue I have with having both in my tank is that they grow together and onto each other. That basically makes it look like a weird mix, but it can't be easily pulled apart since the java moss anchored to the subwassertang and the subwassertang anchored to the java moss. You probably just ended up with a mix of both