The incident occurred after Hicks awarded a penalty to Portsmouth in the 88th minute, which Colby Bishop converted, resulting in Port Vale suffering a 1-0 defeat at their home stadium Vale Park.
Seconds earlier, Hicks had denied Port Vale a free-kick.
The result meant Portsmouth maintained their top spot in England's third-tier competition, while Port Vale sit three points off the relegation zone in 20th.
"We are appalled by the incident ... and condemn the mindless actions of the individual involved," EFL and PGMOL said in a joint statement.
"Match officials should never feel threatened for their safety when playing a pivotal role in the staging of games in our competitions.
"We will provide our full support to Craig and his team alongside Port Vale and the relevant authorities as they seek to bring this individual to justice."
Port Vale also condemned the actions of the individual.
"We will work with the relevant authorities to ensure the matter and the individual is dealt with a zero tolerance approach," Port Vale said in a statement.
"We remind all supporters that entering the field of play is a criminal offence."
