Diego told Maddie, who told Emma, that it releases a person's inner wild - what they wanted to do deep down.
It can make a human sick and pass out.
Mia put the Spider Marking on Daniel's neck, so she can make Emma jealous. She is trying to take down The Chosen One, as it is revealed in Beachside 7. The spider marking made Daniel weak, (which Mia didn't know about), therefore, in El Cristal de Caballero, she decided to remove it off of him.