That was a big surprise to me. I didn't know about this expensive fluid--Honda should have included maintenance costs for this newfangled vehicle in the lease price, IMO.
This post links to an Edmunds article that describes surprise maintenance costs for the Clarity Fuel Cell, such as the ion exchange filter and "a substance called insulating fluid." I also found
this article that describes the incredibly stringent regulations regarding servicing a Clarity's fuel-cell stack and hydrogen tanks. They appear to treat hydrogen as if it were a dangerous gas like the noxious hydrazine used to steer spacecraft!