I noticed this on some of my plants in my mini greenhouse?
Does anyone know what this is? Fungus?

Look at the underside of the leaf with a 30X mag glass or scope and tell us what you see.

Thanks

i don't believe i have a magnifying glass....
but right after i posted i held one of my plants up to the light and i believe i have spidermites!!!
wicked tiny whitish bugs and i'm now noticing the webs....
Do you think they are the cause of this?

I believe they are Eriophyoid mites.  They are almost microscopic.   Of course I could be wrong and they could be just plain Spider mites.  If you have webs, they must be plenty.  Spray them with Need Oil right away.  Will help with Rust or Mites, so you can't go wrong. 

I am also currently dealing with an outbreak that looks similar to this. I have been trying to move everything outside and spray them but the weather has not been cooperating.

in this pic you can see the webbing & the lil buggers

Of course its hard to say without seeing them in person, but those look like regular Two spotted Spider mites.   I would still spray them with Neem oil.  They may be difficult to get rid of.  Keep them isolated until they are under control.

Good luck.

Rubbing alcohol works on spider mites and it's a lot cheaper.

Would you dilute the rubbing alcohol? Make it into a spray?