Typically that's a classic bacterial contamination sign. Though some yeast strains can form a pellicle, a stalactite-like formation of yeast colonies. Really that's only the flor sherry strains, not often used for meadmaking since they're aerobic fermenters. Best suited to sherries and estufa-treated, oxidized ports and the like.
Typically the best way to troubleshoot contamination is to do a literal and exacting task analysis of the process. Like "OK, then I set the spoon down. Where? Was that clean? OK. Then I pour it into the funnel in the carboy... wait, where did I set the funnel after I sanitized it?" kind of thing. When I was new at all this, EVERY bleeding thing I did, before it touched the mead/beer/wine/soda whatever, I triple-thought: "is what I'm about to do going to bring anything into contact that may not be sanitary?" I look at everything in my hand and say did I sanitize that and what has it touched since then? Did I pet the ^%@ dog again?
You know it's funny, these things (trying to pin down contamination) can drive you to distraction. At least if you're a slightly anal-retentive biology-geek child of a public health nurse and neurosurgeon who sees germs attacking his mead at every turn . But seriously, do a mental walk-through, a serious, detail-oriented one, before you begin autoclaving the family cat. Usually it turns out to be something simple like a mouth-started siphon, a hydrometer sample poured back into the batch when you forgot to sanitize the sample jar ('cuz you don't pour samples back in), or the like.
For now, if you're not morally opposed to it, you could consider racking the meads in question out from under the pellicle and sulfiting it to 50 ppm or so (about 1 campden tab per gallon). Many would recommend up to 100 ppm. The thing is that these bacteria will never improve a mead, a very few will cause minor defects, and most will ruin it left unchecked. And the effects increase with time. For sure don't bottle it at any rate; they can often metabolize things yeasts can't and you can get grenades from "finished" meads. Additionally, they can be very persistent in equipment, especially plastic equipment. Most makers of beer styles that deliberately use these "wild" strains of bacteria and/or yeasts (the brettanomyces bruxelensis, brett. lambicus, and the Lactobacilli spp.) for brewing lambics, Gueuzes, and Flemish sour ales, have an entirely separate set of equipment for this type of "biohazard brewing".
The good news is they won't hurt you; no known human pathogen develops in fermented beverages (that's why virtually every early human culture got into fermenting in the first place, aside from the nutritive benefit and the buzz ). It might have funk a mile wide and not make it past your nose, but it wont kill you.
Worth asking at this point, what is your sanitation regimen? Each product, from bleach to no-rinse fancy-pants dodecylbenzene sulfonic acid (Star San), has recommended temperatures and contact times.
Bees, brews and fun
in Lyons, CO