Switching Regulator Ground Moating
Treat the switching regulator as having its own little ground plane - it's almost to be regarded as a stand-alone circuit so my advice is keep the moat around the power supply circuit and don't allow inputting power feeds to the switcher to route via any other ground/rail.

It could be that keeping the moat around the microcontroller has the same effect but as we don't know the full extent of your circuit I have adopted what I feel is the more appropriate way of looking at the problem.