It's still a legal target. Even though Polymorph can't destroy Darksteel Colossus, it will do as much as it can, since the rest of the card isn't dependent on whether it actually destroyed that creature or not. The controller of Darksteel Colossus will still reveal the top cards of his or her library until he or she reveals a creature card.
To confirm and expand upon CarstenHaese's answer, Polymorph can indeed target DSC, since it is a creature and Polymorph's target requirement is "target creature", not "target creature that isn't indestructible".
When the spell resolves (and it will resolve, barring outside interference) it will do as much as possible. It won't destroy the DSC, because it can't be destroyed, but its controller will still get to dig for another creature and put it onto the battlefield.
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Polymorph targets a creature. Darksteel Colossus is a creature, so it is indeed a legal target for Polymorph.
Please use card tags when you're asking a question about specific cards: [c]Serra Angel[/c] -> Serra Angel.
When the spell resolves (and it will resolve, barring outside interference) it will do as much as possible. It won't destroy the DSC, because it can't be destroyed, but its controller will still get to dig for another creature and put it onto the battlefield.