The Jabbawockeez already wear masks, so they will fit right into “Halloween Horror Nights” at Universal Studios Hollywood starting Sept. 18. It’s an “in-between” gig for the Las Vegas-based dance troupe, which closes its resident show at Luxor on Aug. 30 and reopens at the MGM Grand in the fall, possibly even overlapping the last week or two of the California theme park gig, which wraps up on Nov. 1. The break-dancing troupe’s “PRiSM” show is part of the cross-property musical chairs set into motion by MGM Resorts announcing it will build a 5,000 concert hall at the Monte Carlo.