Techsplanation.
As usual this is a GIF because I can't link/upload HTML5 video to Gimp Chat
The video is here:
https://gfycat.com/GrimEnchantingAmurratsnake

It has 364 frames and the shadows on the virtual lava lamp was the hard part.
None of Gimps automated drop shadow effects works on so many transparent layers.

I used the actual animation to make the shadows by pasting it in first to the right and using desaturate, blur and opacity,
and then adding the foreground animation. They synchronize because they are the same animation. I used Wills 'animation merge' but that was also tricky. I used one of the other merge options other than 'normal' (darken only).

I find G'Mic pretty good on multiple layers as long as you give it time to do it's thing.
I also used Gimp-GAP, MathMap and Kdenlive along the way.

I found Wills animation merge is not working on Gimp 2.9 and throws an obscure error message. I went back to Gimp 2.8 for this.

That came out beautiful.