OP, if you read some more around here about infections and how to avoid them, you'll see many references to "cold side" sanititation; this means anything that your beer touches after boil, and the wort has been cooled to fermentation temperatures. I.E., your fermenter, racking cane, hydrometer. Anything that touches your beer before the yeast "take hold" and start vigorous fermentation can introduce bacteria that will cause what you are seeing on your current beer. Even opening your fermentation bucket, or removing the airlock, can let nasty bugs in there.
How long has this stout been in the fermenter? From the pic you shared, I don't see any krausen residue in the carboy, just some dried wort up the neck. What was your sanitation procedure? Starsan (diluted phosphoric acid solution) is the best thing out there for sanitizing your clean equipment before your wort hits it. Others use iodophor but it must be rinsed prior to adding the wort.
Another question; what temperature was the wort at when you added yeast? If it was too hot, it may have killed enough yeast cells that they didn't have enough time to get going before a nasty bacterium took hold ahead of them; you can think of primary fermentation as a race between the "good" yeast and the "bad" bacteria that want to beat the crap out of them.
If it tastes okay to you, you should be safe to keg; make sure to rack under the nasty skin on top and leave enough in the carboy so that none gets in the keg. Per others above, it may change over time. Let us know what happens.