That looks like it is the gasket between the drive flange and the hub rather than the hub seal. If this is the case, confirm by removing the wheel and checking that it is not just spreading from the back of the hub, replacing the gasket is almost trivial, although that looks like maxidrive drive flange and axle, and I am not sure it is the same gasket. 
It may be leaking because the wheel bearings are loose, causing the drive flange to move on the hub, or because the axle breather is blocked, and both these need to be checked. If thewheel bearings are loose, they should be carefully checked and probably replaced, which means replacing the seal.
John
				
			 
			
		 
			
				
			
			
				John
JDNSW
1986 110 County 3.9 diesel
1970 2a 109 2.25 petrol
			
			
		 
	
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