Hello and thank you for contacting Petco Pet Education Center, formerly Petcoach. It looks like allergic reaction, possibly to the grass or pollen. If it is not bothering Milo, if he doesn't want scratch it or bite it, you should monitor him today. Typically, this kind of skin rash should go away within a few days. However, if it is itchy, painful or hot to touch, you should take him to your local vets to get treatment with a cream/ointment or anti-inflammatory injection