I'm new to this forum and greenhouses too. I have a 16' x 20' greenhouse that I assembled this fall. So, I've started some plants from seed and I've bought plugs also. Most everything I've planted and most all of the plugs have had the leaves wither and fall off, and the plants then stop growing and die. I have watered and fertilized, maybe too much or too little, not sure. I have a ventless gas heater and keep the nighttime temperature at least 65 degrees. The humidity is usually 70-80(maybe too high). I have 6 palm trees and a few larger plants that seem to be doing ok. I even have coleus plants that have dropped seeds that have sprouted. I could provide more information, but I wanted to keep this as short as possible. I could use some help. Any advice would be greatly appreciated.

Its hard to say unless we saw photos of the damage. It could be sap sucking pests like mealy bugs, scale or spider mites, it could be watering practices, its just hard to know

Also let us know where you live (or the zone).
And how hot does your greenhouse get during the day?

You didn't mention fans, with the humidity that high at night you could have fungus or bacterial issues especially with the young plants. CONSTANT air circulation will help tremendously.
Find your profile (upper right corner of screen) and fill it out so we can see where you are.

I found some plants with what looks like spider webs on them this morning. Could this be spider mites?

I would say that is a positive id

There can be many reasons for this. I cannot say anything until i saw pics.

There is a pic. It looks like spider mites. You can spray it with Neem Oil or some people use a few drops of dish soap in water with a small amount of rubbing alcohol. But they have pretty much decimated you plant