From what you are describing, sounds like you are looking at the focusing screen. They are usually a slightly frosted plastic with a fresnel on one side and matt surface on the other.
To properly check the prism and screen, look through the eyepiece at a bright, even, light source with the lens off and mirror down. What you should see is an evenly lit diffused field with perhaps a split image or other type of focus aid in the center. If there are streaks or smudges, it could be a damaged screen or perhaps light seal foam gunk on it. The mirror would have to be real ugly for it to show up looking through the eyepiece.
In any case, it would be best to take it to a tech to have a look. Hard to do these remotely. Have him check the foam as well, it goes bad over time.