Rescued fish killed my 150+ shrimp and now got white dots

Hi, I hope anyone can help me and sorry if this isn’t the correct subreddit. my neighbors tank broke a few days ago and I am the only person she knows with a tank, so because it was an emergency she gave it to me. I had 500 liters full of blue dream shrimp, no fish ever in this tank before. The Shrimps got genocided by this blue fish within 48h. Now this guy got white dots all over his body. Is it ich? What to do? First thought was well FAFO for killing my shrimp, but I don’t want to be a bad person, so what is it, how to treat?