Hallgrímsson is Heimir's middle name. In Icelandic, it is common to have a patronymic as a middle name, which is formed by adding "-son" or "-dóttir" to the father's first name. In this case, Heimir's father's name is Hallgrímur, so his middle name is Hallgrímsson.
The Ultimate Heimir Hallgrímsson Quiz: 30 Questions to Prove Your Knowledge