microsoft word
The green wavy line in Microsoft Word is used when Word think you make a grammatical mistake. (The red one is used when you make a spelling mistake) There should be an option in the Word, "Check grammar as you type". You can turn it off/on. Adding some Microsoft links with more information.
The wind's movement of the wavy lines in the sand were fascinating to watch.
That is a correct spelling of the word squiggles (wavy lines or scribbles).
Wavy Red line
When you automatically check spelling and grammar, Microsoft Word uses wavy red underlines to indicate possible spelling errors and wavy green underlines to indicate possible grammatical errors.Blue wavy underlinesWord uses wavy blue underlines to indicate possible instances of inconsistent formatting.
100 words
The tiny lines at the end of characters in some fonts
the french say ondule for wavy
line spacing
The hanging indent marker in Microsoft Word controls all lines except the first line. It allows you to adjust the indentation of subsequent lines without affecting the first line.
It means it (most likely/probably) is a grammar mistake. However, I do think that Microsoft Word (any version) has and can make mistakes with grammar and/or spelling.