Hello Alef, they are sprites popped on the way. Make it fade out and you'll have the marks.
Also, inside the sprite, paint it with semi-transparent black, fading from the edges to the center, finishing in the center with 100% of opacity.
It will be something like this:
Where X is 75% of transparency, 0 is 50% and B is 100%
I'll keep working in this project, also, preparing some class room to Brazilians, where I'll teach and explain how is made a game like this.