Symbolism, Imagery, Allegory
Moose-moss isn't really a moss at all. If you take a look at the illustrations, it's more of a flower, one with blue petals and red stigma. Also, moose can eat more than just moose-moss. They can dine on a variety of different plants year round, including catkins, tall grasses, and water lilies. With that said, just because moose-moss is neither moss nor moose specific, that doesn't mean it can't be a symbol. And that's exactly what it is.
The Lake Winna-Bango Diet
Moose-moss represents the one thing Thidwick has to do for himself. If he doesn't eat, he'll die, so moose-moss is his survival, well-being, and self-sustainability all wrapped up in a flower of a symbol. As such, it's intricately linked to the lesson Thidwick has to learn, which is that helping others is nice and all, but not at the expense of one's mental and physical health.
After Thidwick invites a few freeloaders to live on his head, the seasons soon turn to winter. And once winter hits:
The food was soon gone on the cold northern shore
Of Lake Winna-Bango. There just was no more!
And all Thidwick's friends swam away in a bunch
To the south of the lake where there's moose-moss to munch. (27.3-6)
This is the point where Thidwick's own well-being comes in conflict with the comfort of his so-called guests—if Thidwick doesn't go to the south of the lake, he'll starve (28.2). But Thidwick's kindness gets in the way of his common sense, and he doesn't head to the south side of the lake because the others vote against it. By not seeking out the moose-moss, Thidwick is putting his own well-being—his own self—beneath the well-being of his "guests."
At this point, the moose-moss disappears. What was so prevalent at the story's beginning (the words appear often in the poem and the flower is in many of the illustrations), vanishes at the same time Thidwick's willingness to think of himself does. The symbol and the act go hand in hand.
When Thidwick finally learns his lesson, he kicks those freeloaders to the curb:
And he swam Winna-Bango and found his old bunch,
And arrived just in time for a wonderful lunch
At the south of the lake, where there's moose-moss to munch. (49.1-3)
When Thidwick's concern for himself returns, so does the moose-moss. The picture accompanying this stanza is just loaded with the colorful flower, and it represents the return of health for that big-hearted moose, both in terms of physical and mental well-being.