I've read of Eucalyptus being treated with glycerin and then later dripping and staining furniture or walls. Are the leaves of your Eucalyptus fairly flexible (as opposed to rather stiff and brittle)? If so, I'd bet the glycerin treatment is to blame. I've read that when using glycerin it's important to remove the foliage at the proper time...when glycerin has been absorbed by topmost leaves, then hang upside down to finish drying. I think yours was left in glycerin solution too long and didn't get a chance to thoroughly dry before being shipped out.

I'd return it to craft store...exchange for another batch. Or you might try washing off mold by swishing thru soapy water...Eucalyptus is pretty tough...I don't think it would harm it. Then hang in warm airy place to dry thoroughly.

This is just my best guess..Ive dried my own Eucalyptus without glycern and never had a problem with mold. Good luck. josh