title_body: How can I diagnose dirty taps from drinking a beer?
upvoted_answer: Yes, tap lines that aren't cleaned regularly (every few weeks at least) are susceptible to infection, and you can detect these infections by taste or smell alone.  Common bacteria involved are Pediococcus acidilactici and and Lactobacillus which Tasting Beer describes as having a buttery or "goaty, sweat socks" like aroma/flavor.  I think that probably aligns pretty closely with your "musty" description.  
  
  Is it serious?  In the sense that it can cause illness, no, at least not these organisms.  These are probiotic bacteria similar to what are advertised in some yogurt.  However, given that it does speak poorly to the maintenance routines of this establishment, I wouldn't blame you for not wanting to continue to opt for the draft beer, if for no other reason than that it doesn't taste very good.  
  
  Lastly, it might be worth a quick friendly word to management.  They may not realize that they need to clean their tap lines more frequently, or not realize that it isn't being done.  It's an easily fixable problem and if they do take steps to fix it, everyone will be better off.