Probably some wild yeast or bacteria.
Doesn't matter though, as long as it tastes & smells good.
Normally yeast you pitch will out-compete any other microbes, especially wild yeast.
... Did you clean and sanitize the carboy and your other equipment? Does the carboy have an airlock with sanitizer or spirits? Were all the ingredients pasteurized or sulfited? What yeast was pitched and was it fresh from the lab (if applicable)?
The only thing I did different was I racked the pasteurized cider to the carboy while it was at 170. I put the airlock on and waited until the next morning to pitch yeast, rather than using a wort chiller as I do for beer. I’m guessing this could be bacteria that developed while it was in that temp range for some time.
Cooling from 170F would develop negative pressure and pull in a bunch of air (and contaminants), as well as encouraging wild microbe growth. ... and yeah, stress the glass.
If the juice is pasteurized, it's fine just to use it.
If it's unpasteurized or might be contaminated, sulfites are the best option.
For future reference, I'd suggest not doing that with a glass carboy (it looks like glass to me anyway). You're just asking for that to break, sending shards of glass everywhere.
It almost looks keeved to me... .
I've had cider that looked like that before it started actively fermenting. It turned out fine
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