What is this long thin mark on my lens, fungus or dust?
Unfortunately, the lighting and focus makes it difficult to tell. But just based on the upload photo, it looks like either a fine piece of lint or dust stuck to the inside of the rear element.
As Tetsujin comments, the mark looks like a scratch, but the photos are not clear, so it could be a long piece of lint, as scottbb suggests. It is unlikely to be dust, which are usually small roundish particulates, or fungus, which usually have a branching structure.
If the mark is a piece of lint on the outside of the lens, you can try using a blower to remove it. If it is on the inside and it really bothers you, the inside of the rear element is easily accessible on many lenses. However, I recommend leaving it alone because the of the likelihood of making things worse while trying to remove it.