In internet parlance cancer has become a common insult used to describe something that the speaker thinks is worthless and unpleasant. The allusion to the biological phenomena of cancer is pretty obvious: it consumes resources (usually attention) and contributes nothing of value. It's more insult than descriptive because it's often aimed at people or groups as well as media and trends. Internet cancer is the thing that keeps popping up in your Youtube recommendation no matter how many times you try to get rid of it, the argument that will not die consuming a subreddit you used to like, and every single comment left by that person you should have unfriended from Facebook two years ago but that you know will become a thing if you do. The topic of cancerousness is, of course, subjective and will likely start a flame war if you dropped into a public space assuming anyone even tries to engage with it.