Thats happened to my Frogspawn and Zoanthids before, I think it is zooxanthalle that dies due to lighting changes. Always expelled some every time I moved coral from QT into my display tank, which has a different intensity of light. Nothing has died yet though. Here's a pic.
Disagree Sharko....Bubbles and many other LPS, such as frogspawns and hammers, ect. ect. expell an amount of zooanthellae pretty much daily. Same with many other species. I see my zoo's do this alot as well as any LPS in my various tanks. If the coral appears to start bleaching, then I'd worry, but just spitting up a little un-needed algae every few days isn't going to kill it off.
I would disagree, only state that it is a symptom of stress for the coral - mine has done that a few times - and while it was obviously stressed, it's done wonderfully now many years later.
I would take this seriously though.
Have you changed lighting, is it new?
IMO, I would say this is likely a reaction to a change of lighting [most likely]. I would be careful not to over-light it, and try to determine the cause.
As with most things in a reef tank, checking chemistry [pH, Alk, Ca, SG, temp, nutrients] is always wise with any tank-problem [if just to rule them out].
If you've increased light on it [changed bulbs, cleared water, moved] - I would consider finding a way to somewhat reduce the light it's getting. I would avoid moving it much or disturbing it .... but would be sure you're not going to slowly bleach it.
IME, bubble corals are sensitive to changes ... but if you take things slow, baby the coral a bit - they are very hardy and can adapt to quite a wide range of circumstances [mine went from low light PC to under 250w MH in my care].
I bought the coral more than 2 weeks ago. I saw the bubbles turning a bit brown on almost every bubble maybe 10% of the bubble. It was totally green. Today it is expelling again but not as much as yesterday. Yeah I put it under my lighting directly. It is 4x39W T5.
I would keep it pretty shaded, it seems [IMO] that the green ones may not handle as high of light - and generally these are often used to very low light at importer/LFS.
I'd consider shading it some if it continues.
Generally, if this is brown snot-like stuff, it's expelling Zooxanthellae, perhaps from a lot of light causing some `booms' for it. With these types of corals [I think] it's a major way it can regulate the zoox. counts in the coral if they get unbalanced.
But it is, thus, a sign of stress in the coral if it happens regularly. I'd be sure to keep everything [water quality, etc] in top shape, perhaps slow flow down and try to feed it some mysis/small meaty bits until it's been a few weeks of excellent health. IME, these corals don't feed that much, but very much do feed.