That's Wandering Whistler's hand in the last panel. The misty stuff coming out of the door (and in the air) is just the transitional energy/matter in this "between-universes" realm--i.e., the fuzzy stuff that makes up the Fuzz. The Fuzz at Slip Points (i.e., inside the house) is a bit thicker than outside Slip Points, since Slip Points are places where energies from other universes inside a multiverse seep through.
Since the misty greenish stuff seems to be all over all the previous panels I assumed it entered through the portal just as WW was closing it, somehow invisible to those present but visible to us. I did recognize WW's hand. I think it looks like it is actively going into the door rather than just seeping out as the door is opened because it does not emanate evenly from the entire crack (nor fall to the floor or rise to the sky).
If you've seen the last few moments of this past Sunday's episode of Supergirl,you'll notice Earth-90's Flash looks familiar and the fallen heroes resemble those from Smallville. It's laid the groundwork for the upcoming Elseworlds series,so stay tuned...!
That Stargirl also looks very much like the one from Legends a few seasons ago, and Smallville had no Captain Cold, yet Snart is very obvious. Lastly, Smallville had zero to do with the 90s Flash.
Leaping to conclusions before you have all the evidence is a great way to land on one's face.
Firestorm is also very visible on the ground. Smallville had no Firestorm.
I suspect the entire reference to Smallville is simply a call back and nod to the former show, -not- a hint they will be including it as Canon.
Besides, the Arrowverse has a Supergirl, and she is nothing like the Smallville one.
Leaping to conclusions before you have all the evidence is a great way to land on one's face.
BTW-the actor who played the 90's Flash also played Barry's father and Jay Garrick in the CW Flash series.
https://www.youtube.com/watch?v=0bryH6PVAbo
I suspect the entire reference to Smallville is simply a call back and nod to the former show, -not- a hint they will be including it as Canon.
Besides, the Arrowverse has a Supergirl, and she is nothing like the Smallville one.