My screen glitches and bugs

So I deleted mc from my imac about 2 weeks ago, when I downloaded it again today and joined some servers players turned green and it was like xray ish and somehow I was able to look underground. Random walls and all types of blocks appeared in front of me and I couldn't (still can't) move properly cause I basically didn't know what was going on. Because my description isn't that clear I made a screenshot. I even tried different servers and I also tried different minecraft versions. I own an imac and I'v played minecraft since it came out pretty much