Like so many of the zany characters in this adventure, he may tend to roam around. In fact, the Caterpillar has wandered so far that I can't even find him! Perhaps he has gone in search of a better supply of mushrooms.
However, the Mock Turtle is not so hard to find, but may be hard to catch!
Apparently, "out in the open" is too difficult, so I have raised the difficulty rating. Perhaps this will make it easier to find the Turtle now!