Do money markets exhibit any sort of fluctuations during a stock market crash? Is it any different than placing your money in a bank?
As the commenters have already indicated, money market mutual funds are not guaranteed to maintain principal during all market conditions, and investments in mutual funds are not insured against loss due to market changes.
That said, you can run a price search on Vanguard's website and see these results:
Prime Money Mkt Fund Fund Inception Date 6/4/1975 High High Date Low Low Date $1.00 11/17/2014 $1.00 11/17/2014
So, despite all the economic problems since 1975, VMMXX has never traded at a price other than $1.00.
Wikipedia has a solid article on Money Market Funds which includes a section on "Breaking the Buck" when the money market fund fails to return its full dollar. Money market funds smoothing out the daily (generally small) fluctuations of investing in short-term treasuries directly but have similar risk over longer periods. Some funds can and have lost money in market crashes, though even the worst performers still returned 95+ cents on the dollar.
While few investments are guaranteed and likely none in your retirement account, money-market funds are likely the choice you have with the least fluctuation and similar minimal risk to short term treasuries.
However, a second important risk to consider is inflation. Money market funds generally have returns similar or less than the inflation rate. While money markets funds help you avoid the fluctuations of the stock market the value of your retirement account falls behind the cost of goods over time. Unless the investor is fairly old most financial professionals would recommend only a small portion of a retirement account be in money market instruments. Vanguard also has a set of target retirement investment funds that are close to what many professionals would recommend. Consulting a financial professional to discuss your particular needs is a good option as well.