I'm starting this thread mostly so I can link back to it since I offer this advice frequently. So here is my own advice on dealing with bryopsis.
First, research and see if you actually have bryopsis--- look for the small, feathery tips (even if they look ragged) and look for it to be growing in patches/thickets..... it does NOT have runners like caulerpa. It's usually blue-green or dark hunter green.
Links for identification are found below.
Now, how to deal with it. This is going to be painful to read if you have a large-scale infestation of it.
1) No cleanup crew will predictably eat it.
The odd tang or foxface has been known to eat it. 99.99999% of them (wild guess
) will not touch it. I've NEVER seen with my own eyes any snail, crab, fish, urchin, or other critter that would eat it. It is very unpalatable to most creatures, and the only thing I know that eats it is a hard to find, temperate nudibranch that isn't found in the hobby and I have no idea where I read about it, I just know it exists.
2) No chemical solution is a guarantee.
DEFINITELY TRY the Kent Tech M treatment (buy the specific product "Kent Tech M" and dose 80-100 ppm worth, every day, for a week). This has been known to kill bryopsis, but it didn't work long term for me--- the bryopsis came right back even though I jacked my magnesium up to 3,000ppm (no joke, and the only coral that I lost was a couple heads of torch and frogspawn, nothing major).
3) DO NOT PLUCK IT with the rocks still in the tank water.
Shards of bryopsis can apparently reattach and start new colonies of this crap.
Other than the Tech M listed in #2 above, here are some other treatment options.
4) Full-scale invasions are the most difficult to deal with.
If you have a full tank invasion, try the Tech M. If that doesn't work, I personally just would remove all animals from the tank, frag out all corals VERY VERY carefully (so that you don't get bryo in the frags), and put them all in a holding system. Then bleach-bomb the tank,
sterilize it 100%, and sterilize the whole system. This is painful and a last resort but you aren't the first to have to do this. You can neutralize the bleach with cheap dechlorinator after a 100% water change.
5) If it's on specific rocks, but most rocks look clean, REMOVE the infested rock and bleach it.
DO NOT risk it. No piece of pink, purdy rock is worth having bryopsis in your tank.
6) If it's on a coral, frag the coral and toss the infected part in the trash.
7) If it's in a zoanthid colony, attempt to frag it,
but just know that this stuff is pure hell on zoanthids. I usually throw the zoa colony away if it's more than half covered, but then again I've only had colonies under 100 polyps. Larger colonies might be more salvageable. See #9 for a possible second option.
8) DO NOT SELL, TRADE, OR GIVE AWAY ANYTHING with bryopsis on it.
That is the reef-keeping equivalent of giving your friend a terminal STD.
EDIT: Adding a new option that has surfaced that seems to help.
9) Hydrogen Peroxide--- the cheap 3% solution you buy at the grocery store-
-- seems to work to kill it on frags like zoanthids. Recently I had bryopsis sprout up on a yellow clove polyp frag, and I sprayed the peroxide (full 3% strength) on the frag to reach it deeper in between the polyps, and then gave it a soak in 50/50 tank water and 3% solution for several minutes (up to 5). You must be willing to kill the frag in the process, it's better to kill the frag than it is to risk NOT killing the bryopsis. However, it's been a week, my yellow cloves have opened back up a little, and all seems well-- no bryopsis to be see. I see this as most useful for treating infested frags, but it may hold promise for a whole-tank nuke without fully killing everything in your tank. An act of desperation might be removing any of your non-infected corals (and all fish/inverts) to a separate system, being willing to kill anything in your main display tank, and then start dosing cheap 3% hydrogen peroxide into your tank. I do not know how much would be necessary for a full scale bryo-kill without burning your corals up completely--- I suggest you google around for anyone trying this, or perhaps start with 1 pint of peroxide per 10-20 gallons of tank water (skimmer off during treatment).
----EDIT--- -a few weeks later, that yellow clove polyp frag is healthy as a horse, and has no bryopsis. I've killed bryopsis on several other frags recently, no problems at all killing corals. In fact, I have a yellow-zoa (the long-tentacled polyp type) that as a fast growing sponge overtaking the zoas, so I've soaked it three times (in three weeks) in the 50/50 mix of off-the-shelf peroxide and tank water. 5 minutes per soak. The sponge (which was quite thick) is receding quite nicely, and the yellow zoas are always open within 15 minutes. I am amazed at how little damage the peroxide does to soft corals!!!!