What is the white structure on a grass plant?

The white structure shown here is a ligule. This is the structure which clasps the stem at the junction of blade and sheath. Ligules, being outgrowths from the sheath vary in size, shape, and texture, so they are useful in the identification process. Ligules may be scalloped, rounded, tapered, edged with hairs, for instance, and I have not mentioned all the possibilities.

The ligule is a projection at the base of the leaf blade in the grass family, and is actually a small membrane that extends up the stem of a grass from where the leaf blade separates from the sheath. On the diagram shown in the question, the ligule is white and suggests it might be edible. It will have moisture but not as much as within the stem as a whole.