This is a difficult shot because fish swim past your slider. And because it is a slider, it’s not the best candidate for the remove tool because you can’t ever see “behind” the slider, so there’s no pixels for Mocha to use to automatically replace the slider. Even if there were, the FG fish make that difficult. Your best bet is to use the Mocha Lens tool and unwarp the shot, then rotopaint the slider out, and use Mocha VR to rotoscope the fish back in over the top. You will have to touch up paint the fish where they cross the slider by hand. But the good news is you only have to roto the fish where they cross over your painted area as well. And it’s only two fish I can see in your clip that cause problems.
The lens warp and paint tutorial we have deals with a similar slider. I will link it here: