Looks like you have skin weights of the body that are on the arm... You can fix this by editing the skin weights if its a poly model by painting the weights more using "Paint weight tool" under the skin> edit smooth skin> paint skin weights tool.
If its a nurbs model you can use the component editor and do each point individually.
Kurt
I am enough of an artist to draw freely upon my imagination, knowledge is limited, imagination encircles the world. (Albert Einstein)
https://www.artstation.com/kurtb