I’m not sure anyone knows who Herr Nilsson is. That makes sense given that he (or she) does street art/graffiti. There is an official point of contact and you can find Herr Nilsson’s work in the Keszler Gallery alongside names like Banksy.
There is a monkey character called Herr Nilsson in the Swedish children series Pippi Longstocking. Perhaps Nilsson is Swedish. There’s certainly “children’s book” theme in the artwork before. Childhood friends, cute pictures… but deadly!
You can see more Herr Nilsson on Facebook.