Before suggesting a partner, I'd like to suggest Thunder Wave + Prankster over HP Grass. While HP Grass Murkrow is really nice for luring Golem (on the threat of Brave Bird, Golem comes in and takes a HP Grass instead) and Relicanth, there aren't many Pokemon on this team that struggle with either of these two, possibly making Thunder Wave a better option. Defensive Quilladin certainly takes on both, Swanna checks a Golem that has had its Sturdy broken, Monferno hits both Relicanth and Golem with a STAB CC, and Golem hits each with a STAB EQ, so I would save HP Grass Murkrow specifically for teams that need these Rock-types removed (perhaps when you're using a wallbreaker that likes to spam Normal-type moves, like CB Stoutland or Chatot, or something like Arbok that enjoys its defensive answers being beaten for it before it comes in).
In any case, I think Scarf Rotom-Frost would make a nice sixth here for a few reasons:

Rotom-Frost @ Choice Scarf
Ability: Levitate
EVs: 4 Def / 252 SpA / 252 Spe
Timid Nature
IVs: 0 Atk
- Blizzard
- Thunderbolt
- Volt Switch
- Trick
RawMelon already pointed this out, but you have Swanna Defogging away hazards, which allows offensive Pokemon weak to Stealth Rock to function optimally on this team. What's more, you also have a Murkrow Pursuit-trapping Grumpig, meaning offensive Ice- and Fire-types that appreciate Grumpig's absence will fit nicely in this last slot.
With these two assumptions, and seeing that the team is slower than Floatzel (even with priority included, as Floatzel's Aqua Jet still outspeeds), I think another check to Floatzel is needed; Floatzel's LO or Specs Ice Beam handles Quilladin, its Aqua Jet outspeeds Murkrow's Sucker Punch and Monferno's Mach Punch, its Water STABs beat Golem, and if by chance it is still using HP Electric from an older meta (this is unlikely though), it hits Swanna too. So, this offensive Fire- or Ice-type should be faster than Floatzel and be able to check it at the same time, which narrows our options down to Scarf Rotom-F.
Just like Murkrow, Rotom-F will place a large amount of pressure on Grumpig, and will weaken it with Volt Switch or Thunderbolt (along with the team's entry hazards), meaning Monferno will be more able to clean up late-game with Mach Punch.
And lastly, not only does Rotom-F supply the team with some much needed Speed, but it also provides it with resists to Electric- and Flying-type moves, which means you can play with Golem early-game as recklessly as you'd like without the fear in the back of your mind that sacrificing Golem early on will leave you vulnerable to said types later on in the battle.