Masking a mothership might be an issue, especially on the scale discussed here.

The equations for reaching a fraction of the speed of light are fairly simple (specifically, without considering warp-drive techniques). Building a working system is undoubtedly complex, but it's chiefly a game of attaining the required energy.

Cloaking a large stellar object so that it's invisible is a much more difficult task. There still might be ways, but spacetime should see clear warping at the given masses and velocities.