You shouldn't have the same folder in name in both seamonkey folders.
Obviously, the "Seamonkey" folder (capital S) is the main folder in your Home directory. Now, in Seamonkey should be the extracted "seamonkey" folder containing the entire package (chrome folder, etc). If it isn't thecase, then extract to that first "seamonkey" folder (lower s).
Open "Seamonkey", and right-click on seamonkey -> Open in Terminal.
Then type ./seamonkey
Otherwise, if the current UM is a fresh installation, then install the entire system again and then do it as I described.
PS: Make sure that you really created the folder Seamonkey via Terminal, because it seems that you might have a permission issue, looking at https://duckduckgo.com/?q=failed+to+execute+file+seamonkey&t=h_&ia=web