story identification - 80s kid's book about colonists on a planet with large creatures

I'm struggling to recall the name of a young children's book (maybe ages 8 - 10?) about colonist on another planet. I read it in the mid 80s but it might have been older than that.

The planet had large creatures which hid in tall orange grass. For some reason I think the grass was called Zork Grass or something like that, but I don't think the book had any relationship to the Zork computer game, though I could be wrong.

I can see the picture on the front cover in my mind as clear as can be. The creature looks kinda like the ghost in Mario: http://www.mariowiki.com/images/thumb/a/a0/Boo_Artwork_-_Mario_Party_Island_Tour.png/180px-Boo_Artwork_-_Mario_Party_Island_Tour.png (without the tongue). A white body, kinda featureless face, huge mouth with jagged teeth.