He said witnesses saw Mogan throw chewing gum at Semenyo, hitting the right side of his shorts.
In a statement read to the court, Liverpool supporter Neil Sherratt, who sits next to Mogan at matches, said he saw him approach the pitch from the disabled section of the Main Stand.
Sherratt said he heard Mogan shout something along the lines of “hurry up” and saw Semenyo turn towards him.
He said the footballer had a shocked expression and he believed he replied: “you what”.
Another Liverpool fan, Nicholas Collins, said Mogan became “very cocky and brazen” after the incident and threatened to assault police if they approached him.
Mogan, who had an intermediary with him in court, told police during an interview the following day that he had shouted at Semenyo to hurry up.
He claimed the footballer had called him “a cripple”, which had upset him, and said he had thrown chewing gum on to the floor rather than at Semenyo.
Jones told the court the prosecution’s case was that Mogan had lied during his police interview.
Semenyo is expected to give evidence over a videolink.
Mogan, of Templehill Close, Dovecot, denies racially aggravated behaviour.
The trial continues.