it will just be underpowered but no it will not mess anything up, if it were the otehr way around that could damage your sub. The best way to get the best sound out of a sub while giving it less power is go ported or a large sealed enclosure.
you WANT to have a lower amplifier output than your speakers' power handling.
the amplifier RMS power rating is actually how much power the amplifier puts out at peak output.
the RMS power handling rating of a speaker is only a thermal rating for how much heat the voice coils can handle without getting damaged. it doesn't mean you need that much power to reach peak excursion.