Mike saying "I'll stay with Will" but he's possessed by the Mind Flayer.
my morse code theory below is batshit b-b-b-bonkers, stay tuned.
In S5E6, Mike oddly taps after he stays behind with Will, who's lost in Vecna's mind.
Simply, perhaps this is due to Mike nervously twitching. *It could be something Finn, the actor, does because he's also a musician, or because he's counting to keep pace with the scene (however the scene is fairly simple and this tick would've been addressed during filming). However…
Tapping morse code is what Will did in S2 while he was possessed by the Mind Flayer.
It is also something prisoners of war do on cells/pipes, but there is a special system called the "tapping code." If we go by this method, he's tapping "FF" (which I've only seen modern gamers use as "forfeit"). The show didn't use this version, instead they just used morse code.
Mike makes three flinching bursts each hand. You can decode the speed of the movement or how hard he's flinching (it's harder to guess how much pressure he's using (we can't hear).
With speed, my eye is seeing "UD," his righthand taps twice fast, pauses for another tap. His left taps once then pauses for two more taps. He signals UD as in Upside Down.
With the severity of the flinches, his right hand flinches quickly. His left hand is stronger each time. Spelling out "SO" as in an S.O.S, a distress signal for help. Later adapted as "Save Our Ship" (BYLER).
Regardless of what the tapping means, the callback to Will's possession is suspicious. It could mean Mike is currently possessed or held prisoner in Vecna's curse signaling for help from the Upside Down.