Marble and Onyx Fish Decor

This piece is something I found to be quite different from any I recall. From Mazatlán, Mexico (Etched on the tail fin) He's heavy, 5 pounds an 9.5" tall x 8" long and the base is 3" diameter.

and here's cm to make it easier for some:

24.13cm tall x 20.32cm long x 7.62cm diameter base. 2.26kg

A screw attaches both pieces together and I'm not sure if it's meant to move around or it just loosens itself because it seems to come almost 'loose' a lot. So, how are the black pieces and the marble put together? I know they are separate by looking closely and by feeling it, you are able to tell. Besides of course the color.

Here's my dilemma; on one side of the fish, only on the black, what I believe to be onyx (?), there are what seems to be water spots. If this has any type of 'glue' then I can't soak it. I did use some soap and water and quickly wiped it off, so I know it's not something I got on it. (wasn't sure because of course, I just saw it!

I don't know what would take this off? Does anyone have ideas to remedy this?

Would love to hear them!

Thanks for your help ;)