The non spoiler tip: A book in the library has hints on that.
A bit more forward tip:
You need to bring it back into the Mermaid Tower where you got the mermais TF.
how get past barrier
You refer to the one at the entrance of the tower?
If so: It gets removed after you completed the story relating to the deep sea pearl. Just as with the mermaid you need to enter the tower from the top.
