Yellow rubbery underseal....
 

Any easy way of getting this stuff off the steel?

Its incredibly resilient, The only way Ive found is to burn it with a blow-torch then scrape it whilst its still burning. It Stinks and cant be good for your health, let alone the fire risk!

Any easier way?

Try some ammonia. Stinky as all get-out but it's pretty good at breaking loose adhesives.