Commission Mai

There are two maybe three things she'll probably think. (Thinking within the bounds of her character.)
 
1.) That pebble did a bit more damage to her foot than she thought. (Nothing severe but just a tiny cut or something.)

2.) It was some blood-sucking insect that was at her foot and was simply crushed under her right foot.

Or

3.) A small piece of some red berry somehow got in her boot.

All options would probably give her some slight and minor disgust before the red spot on her foot was simply being wiped off and forgotten about. Doubt she would even give much thought or a closer look to find out it was a tiny man that was in her boot and against her foot.

Edit: On a closer look on her face, I think she noticed the dead tiny man. She's probably completely disgusted by it and probably questions to herself how it even got in there but she definitely wants it off her foot as soon as possible. Probably along with lengthy foot bath to relax and forget about this whole thing.

Also, I like the added detail of putting black nail polish on her toenails due to the black nail polish she canonically wears on her fingernails. (Which is actually kind of rare to see anyone have in the series. In fact, I think she's the only one to do so. Don't know about Legend of Korra, haven't taken the time to sit down and see it yet.)