Explain what Don John meant when he said, “I had rather be a canker in a hedge than a rose in his grace,” and to whose grace is he referring?

Don John is talking about his brother who he hates. He says he would rather be a thorn pricking him and forever causing him stress than a good brother. 

'his grace' is referring to his brother Don Pedro who is the Prince and he is saying he would rather be a flower with a disease than being beautiful with his brother