Thanks for helping out a fellow player! Good on you for figuring it out, and sorry if it was a pain to find.
I tried to signal the location of the item with a couple of cues: it has the most complex puzzle in the area, and the outline of the sprite on the wall is the darkest color in the palette, unlike the rest of the display items. (In fact, I use the darkest color in the palette throughout the game to signal interactivity, except in underwater areas, where I use the lightest color in the palette.)
That clearly hasn't been obvious to everybody, and I will absolutely keep that in mind on my current development project. Thanks so much for the helpful feedback!
Viewing post in Taiyaki Fabulous Museum of Fish comments
I think what might have worked against it was the rest of the similar "placards". There's a number of them around he game, and of course lots of them in the museum, and every one of those when you read them after solving, they just give you a bit of info about the thing that you just solved. So by the time I got to that last one in the museum, and spent a while solving it to see something fairly obvious on the wall, I never even considered that looking at the placard again would provide anything beyond "It's a diving mask", and maybe some commentary.
To be honest, I didn't even think to interact with the money bag the first time after I solved that puzzle because no solved puzzle I'd interacted with up to that point, with the exception of the teleport leaf (which I hit by chance), and the chest (which was obvious) had done anything beyond just providing some flavour text (and I don't even remember if the chest needed a second interaction, or if I got the shovel immediately upon solving it now - which means if I did need to interact with it a second time, that being a game mechanic didn't actually stick in my mind)