My youngest 7 years old cat Neo, grey in colour, came in this morning with this yellow green stuff all over his fur on his face. 
Looking more closely, it seems to look like its been done with a yellow highlighter pen!!
I am not at all happy to think some one has wrote on my cat, more to the point is he going in some ones house with kids where they're taking the mick out of him?
Sorry to rant, but I'm tamping

Looking more closely, it seems to look like its been done with a yellow highlighter pen!!

I am not at all happy to think some one has wrote on my cat, more to the point is he going in some ones house with kids where they're taking the mick out of him?
Sorry to rant, but I'm tamping

My cat often has a turquoise face because he loves the smell of my highlighter pen and will fight to rub himself on it. I stop him because it can't be good for him but so far it hasn't done him any harm. Maybe your cat found a discarded one.