Yes, MOLTEN is a Scrabble word!

Letter values in Scrabble US: M 3, O 1, L 1, T 1, E 1, N 1.

Is it a valid word?

MOLTEN is a valid word in the official Scrabble dictionary (US), the UK Scrabble Dictionary and Words with Friends (ENABLE).

reduce or cause to be reduced from a solid to a liquid state, usually by heating
become or cause to become soft or liquid
become more relaxed, easygoing, or genial
lose its distinct outline or shape; blend gradually
become less clearly visible or distinguishable; disappear gradually or seemingly
become less intense and fade away gradually

Also: melt down, mellow out.

Not playable in Scrabble US: UNTHAW, DETHAW.