Illegal Pitch

[NM FP Ump]

USSSA 16U.

I am PU. After receiving the ball from the catcher, the pitcher bends down and picks up a bit of the chalk from around the circle. She wipes the chalk off on the side of her shorts and prepares to pitch. I think, no problem, she's wiped off the chalk.

Next pitch, same routine but the pitcher now works the chalk between her fingers and the excess falls to the ground. Now I'm thinking, she's pushing it. I don't give the illegal pitch signal until after the pitch. While tracking the ball I see a white blur on the ball (she threw inside, so it was easy to track). I call "illegal pitch" and warn the coach that his pitcher cannot apply a foreign substance to the ball. He didn't argue, just told the pitcher to "wipe it off next time."

Two questions. First, should I have called IP on the first pitch?

Second, do you think chalk is a foreign substance? I can see both sides of the answer. Yes, it is a foreign substance because it is brought to the field to establish the lines of the field, which I agree with. But I may also be swayed, after further thought, that chalk is close to a resin bag and should not be a foreign substance, since a resin bag is legal (Rule 7, Sec. 1. H.).

Feedback, please.