The entire prop has three layers - the inner metal hub that is in
direct contact with the engine's prop shaft, the outer prop itself, and
a middle layer which is made of hard rubber, is bound tightly between
the two layers, and is designed to break free if the prop impacts a
solid object - to protect the inner gears and such.

"spun prop" means there is slippage between the rubber hub and the
other part(s). Bottom line is, in the water, the engine will turn the
prop shaft, but the prop itself slips due to the load of the water.
That's why it will be more obvious at higher rpms in the water as
opposed to idling or on the hose.

Specialized prop shops have equipment to install new rubber hubs. This
is worthwhile on larger expensive props. Your 20's prop is probably in
the vicinity of $75 or so, so it may or may not be worth while -
especially depending on the overall condition of the prop. If it's
dinged up, it's probably smart and cost effective just to buy a new
prop.