What does the note from chapter 14 say? Here's the note from the last chapter:
Utter this incantation!
  If you don't, you won't be able to see mermaids.
  
  ↓
  
  Mermaid, mermaid
  I beg of you
  Make my love come true
  For me, [do something with] blood and flesh
  
  (end of incantation)


(I know nothing about this manga. The last line of the incantation has an implied verb; one would need context in order to determine what verb should go there.)