While it often tends to be easy to figure out today which class a devil fruit belongs to by just seeing how they work without a true confirmation (like with Urouge or Loki) what class Prince Grus' devil fruit has always been confusing in the sense that it's unknown if it's a Paramecia or a Logia but I came to realize that it's the former because of two reasons.
The first one is that it's actually listed as one in the vivre card books
The second reason is how he uses it, if it had been a logia than I don't doubt that he would have used I similar to Caribou uses the Numa Numa No Mi which he didn't but I actually came to realize that the way he uses his devil fruit is extremely similar if not identical to how Galdino (Mr. 3) uses his Doru Doru No Mi which is definitely something you can't unsee once you see it.
Here's a list on the similarities:
Both can harden their the material they control into object or a blunt weapon
Both can create massive quantities of the element they control
The way they control their element is similar
So that's it, it's a paramecia