First ask yourself if it's ...

First ask yourself if it's really a problem.  An inch to 2 or even 3 inches of foam, especially if not fully covering the tank is not really a problem.  As Tom mentions surface wasting (skimming mixed liquor to WAS) essentially acting as a "mechanical selector" (to select out nocardia) is the best way to do it, if you can do so with the set up you have, or could reasonably have, with a few relatively minor modifications.  Lowering MCRT may help but lower MCRT's often lead to more WAS and may make nitrification difficult as noted by another comment, assuming you need to meet a nitrogen goal.  The last resort is chlorination applied either to the surface through foam sprays (add hypo to the lines feeding foam spray system) and or by adding hypo directly to the RAS.  The latter may impede nitrification and or harm other desirable microrganisms so you should start with low dosage (7-10 mg/L on RAS Q) and gradually increase til you either see problems or achieve nocardia reduction.  Some would say if it's not deep enough to pose a risk of overflowing the aeration tank then it's not a sufficient problem to be concerned about.  You definitely don't want to allow it to build up to that level and believe me some plants have experienced that big a nocardia problem.  So don't sweat the small stuff and do go through progressive steps to control these nuisance microorganisms, starting with a mechanical selector first if you can set your plant up for it.