Well its time to ask the "I" question

That's a lot of headspace for "secondary" (clearing vessel), especially after a month in primary. Are you in the lagering phase now? Did you do the diacetyl rest still in primary before racking? It's weird for anything like that to form at lagering temperatures.

The reason I ask is because once fermentation ends and the diacetyl rest is done, it's good to go ahead and rack the lager and begin lagering. I know many people like to recommend a month in primary, but once the beer stops producing co2, racking it to another vessel with a lot of headspace like that leaves it vulnerable.

Hopefully it IS just a change in the weather, but it's impossible to tell in that picture.

 

Well, not really. I mean yes, of course it won't be a maibock since it's not a lager. But it should still be tasty.

I'm really concerned about that huge amount of headspace since primary was finished probably about a month ago. It's generally ok in the original vessel, but racking that long after fermentation ended and then having it sit at room temperature with that much headspace leaves the beer vulnerable. I've seen about 4 infections posted just this week from people with the same MO.

I would suggest that if leaving the beer in primary for a month, to package it at the end of that period instead of racking it and leaving it like that. Since it's too late now, I"d wait a few days and see if a pellicle formed or not. If not, I'd bottle and drink ASAP. If a pellicle does form, you could wait and see if you get a nice sour out of it.

 

That's good, then!
It's really hard to tell in those pictures but your description of "Smell is quite sour for what Im used to at this point of brewing. Does show the kinda filmy/milky marks on the side of the carboy slightly above the liquid line" makes me consider lactobacillus.

 

It would get mildly sour at first, worsening with time. If it IS lacto, and it starts to sour, drink fast! It won't be unpleasant at first, but with time it'll get more and more sour and the bottles will start to gush when opened. It's not a "vinegar" sour, though. Vinegar is distinctive- and lacto is a bit "softer" sour, more like a gentle lemon juice. Lactobacillus is a group of bacteria strains which includes pediococcus. Sometimes "slimy" makes me think immediately of pedio infection.

Aceterobacter contamination makes vinegar, and what happens there is the bacteria eat the alcohol and produce vinegar so you end up with an alcohol-less vinegar. That's fine if you're making malt vinegar, but not if you're making beer! But like I said, it's a definite "vinegar" note so if you're not getting that, it's not acterobacter.

Brettanomyces is a yeast that is used in gueuze and other sours. It's described as "horse blanket" or barnyard. It smells really dirty and sweaty to me, though!

If you DO get an infection, make sure you bleach bomb any glass and reusable equipment and toss any plastic that is in the brewery. Once you get something like this, it's something that can continue if you don't take steps to get rid of it.

I had a lacto infected California common a few years back. It tasted sort of weird by day 14 in the fermenter, and by day 21 it was distinctly sour. I used that fermenter (plastic ale pail) for grain after that, and tossed all of my plastic tubing. It never came back, but I was worried for a while!