Grohk using maelstrom can farm easy damage off of your team using Luna. Your best bet is to hide Luna behind a shield if you're capping the point with Luna. With totemic ward, he counters Luna's rush and your ult.
If Grohk is using totemic ward, he does have cc immunity against your ult and rewinds, but you can just 2 tap his base totem without buying bulldozer. If Grohk is using maelstrom, your stasis field eats his shock pulse, preventing the spam.