I've noticed a few references to altocumulus clouds in GF (i know i'm not the first one, i saw someone else mention it)
For one, the "secret society" mentioned in Mystery Shack Mystery was called the "Royal Order of the Holy Mackerel". A 'mackerel sky' is a type of altocumulus cloud formation (it is also a very common cloud formation, i saw several today)
it should also be noted that "Holymackerel" is Alex Hirsch's Deviantart account.
[I also have confidental information related to this and one of my other GF
Someone mentioned that part of Stan's tattoo resembles a symbol for an altocumulus cloud.
some altocumulus clouds resemble UFO's, and some people believe they are
In other words, I'm 99% sure that Stan is a member of the Mackerels.