It probably is mold, Erica. I'm in a high-humidity area, & have found mold growing on some of my drieds. If the humidity is getting above 55% very often, it will probably be too damp. If the mold is not in a conspicuous area, maybe you could remove the affected leaves, & move the wreath to a drier area. On the porch, a grapevine-based wreath might be better, decorated with something not so apt to be affected by dampness. Maybe bittersweet berries for fall, or rosehips. Fall leaves can be dried & dipped in melted paraffin for extra sturdiness, then glued to the wreath base. Artemesia doesn't seem to be affected by the dampness too much, either.

I, too, have experienced a problem with my dried eucalyptus wreath that I bought only a month ago. But, my problem is that it's weeping eucalyptus oil! I couldn't figure out where the yellow oil was coming from as it was in spots on my white door and around the frame. Then, I looked at the wreath and could see droplets on the ends of the stems! I'm not sure if it can be salvaged or not. The door is sheltered from the elements, but we've had a very damp fall and it does get windy. Any suggestions? I would take it back but I lost the receipt from Michael's.

https://krostrade.com/blog/how-to-get-rid-of-mold-on-dried-flowers/

Sulfur dust should do the trick but don't keep in an area that humid