Judging by the contact colour of green you forgot to set the precursors to hostile. Go to the alien races (intelligence?) screen, select whatever the precursors are called and check the hostile box. The contact should appear as red on the tactical map.
Once you do this your units will react to hostile presence. Defending units will fire back and attacking units will seek and destroy. Right now the precursors are set to neutral.
i seem to be having an issue, perhaps I did this wrong. after landing ground forces for over a year of game time I've taken nothing but losses. having fallen from 500kt down to less than 50kt remaining. fighting a precursor force of 3700t (unless im reading this wrong) having destroyed 0 tons (unless im also reading this wrong).
how do I launch a ground invasion and get my ground forces to fight back?