Probably not that unusual for any power tool, a drill, grinder, polisher. Heat, caused by both use, and by sitting in a warm place can cause greases to liquify and separate a bit.
It's not like the Flex Tool has an Oil Pan, or Transmission, blew or leaking at a gasket or seal. and the tool is now going to run dry and burn up like your car.
Just wipe away the oil and continue on. If you start noting that things are sounding or feeling different, then there might be room for concern and a need to operate.
Even the best synth greases, with temp range from -65F to 450F still can separate and get funky with heat and time.