It's hypalon. Great little boats. Bought an Avon 8 myself tail end of last year as a tender.
To clean, then try a green pad, hot water and either Fairy Power Spray or TFR (traffic film remover). Wear gloves because the TFR is a skin irritant. You can protect tubes with 3M inflatable boat cleaner if it's inflated all the time.
Stubborn areas, then you can lightly wipe with MEK on a cloth, rinse off, etc.
Sula
