It was definitely a slider. Check out the vertical and horizontal movement (sort by Ray then order by velocity): https://baseballsavant.mlb.com/gamefeed?date=2021-03-19&gamePk=642099&chartType=pitch&legendType=pitchName&playerType=pitcher&inning=&count=&pitchHand=&batSide=&descFilter=&ptFilter=&resultFilter=&hf=pitchVelocity#642099
Didn't have as much drop as some other sliders so maybe it was firmer than he wanted it. Not a fastball or changeup though.