Bryopsis. Some of the hardest to remove species of macroalgae encountered in the hobby and 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. Battle by:
1. 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.
2. Starve it out - if you can lower nutrients nuisance algae has a harder time taking hold, or coming back after manual removal. Reduce white lights also.
3. Repeat every time you see a little bit come back. Try to be aggressive when removing these species.
Once you reduce it, adding the following will help:
urchins, Emerald Crabs, chitons, and even the larger astrea snails and cerith snails