What is this?

[RSM]

That looks like it, only mine is green. Thanks for the link.

[Myka]

Yep, that's Sargassum...yank it. It will grow like a big, bad, ugly weed.

[RSM]

Consider it done! Thanks.

[Myka]

No problem! I fought with Sargassum in my tank for a few months. The stuff will keep coming back.

[seanoman]

we have tons of that stuff growing on our rocks too. It's a real pain to try and get rid of it, especially if you have a new system like ours. I sure hope that it dies off soon. Good luck!