Shannon, not sure what they may have used since none of the Hohners I own have that adhesive, more than likely the previous owner may have just used some adhesive he found at the hardware store. I don’t know if you can scrape it off with care without damaging the Melodica. Take some pictures so we can get a better understanding of the condition. Plus I really want to see the model you have. I have never seen a Professional 36 with reed plates and would like to hear it to compare to the individual Reed one.