Is this in a 401(k)? There's usually a way to set what new contributions are invested in, so my guess is that the money market is just the default since, as
@seattlecyclone says, it's basically cash.
For me, in Fidelity Netbenefits I click the plan name, then click the Investments tab, then click Change your Investments, then click Change Investment Elections. From there I can click a Select Funds button at the bottom of the page to select percentages of available investments to put future contributions towards. I don't know how universal this is across Fidelity's workplace plans, but maybe it will at least point you in the right direction.