Crying at breast and slimy green poo

My baby is 10 weeks and is gaining weight well. He’s happy most of the time and content. But when it comes to feeding he screams at my breast and won’t latch on and when he does it’s a long battle to get there, he’ll then scream continuously constantly latching and unlatching for 5 mins max and then he’ll be finished. I have no idea how he’s gaining weight but his poo’s are completely mucus, slimy and dark green

HV told me to cut out dairy for 4 weeks 10 days and no difference yet. Should I be worried m?