You can call it a "stepdown", no skin off here. The thing is, though, the term step down also refers to another electrical device, a transformer that lowers voltage. Don't be surprised if the term causes confusion.
No real reason that the 50 amp receptacle can't be used other than there are two extra connections. Some purists would object but the 50 amp receptacles are used less than the 30 amp receptacles so may be in better condition. Depending on the owner of the CG, there may be some issue, some CG's like to upcharge for 50 amp service.