"Molten" to me implies it is still liquid. Molten salt reactors, molten magma from a volcano, molten sand, molten steel, dipping something into molten cheese. All fluid.

If I was to nitpick, "melted" is kind of inaccurate and not entirely natural in this context. Technically, molten sand is also melted sand, because that's how you get it to that state? Usually, you'd hear about solidified magma, crystalized sand, cast iron, air-cast steel, unevenly settled corium... to make a better point on how it turned back into a solid and what to expect from it - something like "The molten sand crystalized into an unusual structure" would be clearer.

I'd usually rather hear "melted" if it is important to note that this had a phase change and back. Plastic on an electrical device may look melted, indicating heat. A hardened steel part may look melted, which may damage the hardening. Rubber on a hydraulic line may look melted, also indicating heat. A plastic container looking melted in the context of chemicals may indicate some compromise.

Now the words sound weird in my head. Thank you.