I’ve an 82 300D. Today, when changing my hoses, I noticed that inside of the top radiator hose a rubbery deposit that I mentioned above. Any clue, anyone?? And, i need to do what to correct this?
Btw, the coolant was green. However, my MB garage mentioned I should switch to G38.
Any thoughts would be appreciated.

Just at the bottom of the hose, or did it coat the full inner diameter?

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It wasn’t everywhere. Just at the top and about 4” into the hose

Is there a lot of buildup? Seems like there's usually a bit of sediment at the bottom of hoses, the bottom of coolant reservoir bottles, etc. Slow breakdown of the coolant, dissolved minerals (always use distilled water whenever possible), and corrosion washed from inside the head, the block, water pump, hoses, etc. ends up in eddys and low spots in the cooling system. As long the sediment isn't substantial, it's probably nothing to worry about in an aging car.

Be more concerned if there's oily gunk floating on top of the coolant, which indicates a failing head gasket.

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Thank you for your reply. It was as posted, like a rubber buildup on the inside of the top hose. Thankfully- no oil in water or vice versa.
I'm replacing all hoses now, just in case.