I think you may be misinterpreting what you see. The otto can more than escape the shrimp if it is so inclined.
If you put your hand into a tank full of ghosts they will actually give you a "manicure" and trim away any dead skin. If the otto stood for this type of treatment from the shrimp without trying to get away it is likely it had some diseased tissue on its tail already, like fin rot or just damage from transport. Often new fish from your LFS will have problems like this due to the stress they are under.
I wouldn't worry about it unless you see the otto fleeing the shrimp and they can't manage to get away. An otto that can't escape a ghost shrimp is probably doomed anyway.
If you put your hand into a tank full of ghosts they will actually give you a "manicure" and trim away any dead skin. If the otto stood for this type of treatment from the shrimp without trying to get away it is likely it had some diseased tissue on its tail already, like fin rot or just damage from transport. Often new fish from your LFS will have problems like this due to the stress they are under.
I wouldn't worry about it unless you see the otto fleeing the shrimp and they can't manage to get away. An otto that can't escape a ghost shrimp is probably doomed anyway.