Its' bryopsis. -
Some of the hardest to remove species of macroalgae encountered in the hobby are B. pennata and B. plumosa. These two species have noticeable discernible midribs (center portion of the algae), that are wider than their branches. They are fast growing, and form a mat like root system on the rocks. Algae that grows from mats, instead of singular holdfasts, are harder to remove if they spread in your tank. This is due to the tedious work that is required to remove all of the algae from the surface it is growing on. Any piece that remains will likely continue growing.
they are more sparse branching than its closely related cousin B. plumosa which has more symmetrical and fuller branching. There are many, many species of Green Hair Algae that have feathery branching, and are not necessarily members of the Bryopsis genus, because the hair algae in your system has branches does not mean it is one of these algae species.
Pitho Crabs, urchins, Emerald Crabs, chitons, and even the larger Astra snails will eat both of these species, but often do not consume it faster than it can grow, or the algae lingers half eaten.
If you do confirm you have Bryopsis, help out cleaners by:
- Removing all you can by hand. Just be careful about it, and if you can pull the rock out to remove it all the better. If it has taken hold in the sand, sift it out with a net. If you don't remove the rhizomes (roots) it will grow back
- Starve it out - if you can lower nutrients nuisance algae has a harder time taking hold, or coming back after manual removal
- Repeat every time you see a little bit come back. Try to be aggressive when removing these species
