Diet My Piggies Are Picky!

So I have two guinea pig males, Yogi and Bear who are one year and two months old. I have been feeding them the same stuff since I got them, and it seems that was a huge mistake (I am a first time owner). Today I found calcium build up in their cage and learned they are apparently getting too much calcium? I will attach the picture of the build up I found below, I’ve never seen anything like that before!. Anyway, I’ve tried several times to change their diet but they refuse to eat anything other than lettuce and carrots. Anything else and they lift their noses up and walk away! I try to add a little of the new veggie into their regular but they pick out the new stuff and toss it to the side! I don’t know what else to do for them I don’t want them to get bladder stones! This was what I found in their cage: