Yes, it may be replaceable, but on a Canon 550D it’s generally not an easy user-service part. A Canon service centre should be able to advise or replace it, but it’s worth considering the cost versus the benefit.

If the scratch or dust is only in the optical viewfinder/focusing screen, it will not appear in your photos. During exposure, the mirror flips up and light goes to the sensor, so viewfinder marks don’t affect the recorded image.

DIY replacement or cleaning is risky. These screens scratch very easily, and the retaining parts can be delicate. Unlike some higher-end bodies, this model isn’t designed for simple user-swappable focusing screens.

If the camera is otherwise working fine, the most practical option is often to leave it alone and keep shooting. If the mark is too distracting, have Canon or a qualified repair shop inspect it rather than trying to clean or replace it yourself.