If you use the After Effects then you need to use Mask Footage for creating alpha channel.

If you can’t do this or if you have any questions, please write me an email using the form on the right of my start page. I’ll be glad to help.

Thanks for your feedback!

Unfortunately, I can not do much to change the stony corals, it is the edited photo. But I can only change the animated algae and the tentacles, because they were made in 3D animation.