It's detergent.Oil all had it years ago.It does not effect the lubricity of the oil.

 

The surfactant detergent in oil was petroleum based in engine oil not water based like in Dawn or Sunlight

 

A scraper in one hand, and a shop vac in the other... I'd still be afraid of the chunks that got loosened, but didn't quite come off. Then they break loose while you're driving.

Today's oils have lots of detergents in them. The gunk may slowly get cleaned up with regular good maintenance.

You could power wash the valve cover while it's out of the vehicle.