Rho is the name of the greek letter that he is using. It actually looks more like a stylized p:
Here it is typeset using LaTeX
[tex] \rho [/tex]
Here it is in text
When handwriting, people often add the curly bit to the stem in order to distinguish it from their handwritten p's. Incidentally, I've also seen a variant of rho in some documents that *is* indeed curly.
You could try copying and pasting the latter version into your Google Doc. Whether it will show up depends upon whether your computer recognizes the character encoding being used.
IF you were using MS Word, then you could go to the menu bar and:
Insert > Symbol...
This will bring up a character map that includes greek letters. I don't know instructions for google docs sorry. It's worth a try though, because some of these freeware programs are deliberately laid out in direct analogy to Word.
Note that the use of rho for density is fairly standard in physics.