I can't remember the name of it, but there is a special type of plastic glue that is thin, and when applied to the crack will soak in, NOT superglue (superglue does not work on plastic).
The only danger of the crack is that if it opens up a bit, some skin can get in and the pinching could be very painful.
What are you using for lubricant? Most oils are fine with plastic, but there could be an additive that is reacting with the plastic and making it brittle and crack. You should consider changing the type of oil you use. Or if you are using a lotion switch to oil.