Just noticed this thread as I was searching for some halymenia for my sump. From what I've had previously, halymenia is soft and slippery.
Here's a good ID page. There's a "Search for:" textbox at the bottom.
http://www.algaebase.org/PictureSearch.lasso?
I guess they both look the same.
Here's what some species of Halymenia looks like:
