I like the idea in general, after all the mycus tries like hell to seduce the player as well.
I’m not a fan of pigeonholing them as adopting religious trappings. I do think that is AN option, and I think it would be neat to even come up with a rough outline of a religious doctrine for them that sincerely tries to set the stage for a conversion (adapting to change, taking things as they come, downplaying the aggressiveness of fungal incursions, etc), but committing to that leads to them being super obvious to spot for any player who has encountered them before, and will make players that aren’t aware of them fairly suspicious right off the bat due to the strange behaviour. I think some of them, probably most of them, should present themselves as ordinary survivors, at least at first, and only switch to proselytizing once they’ve gained some trust in the group they’re infiltrating.
Also I think a large number of them, likely the majority, wouldn’t even push the religious angle. They would just join a community, and either take leadership or make themselves indispensable. Then they would simply work to subvert key individuals by any means necessary, culminating in voluntary, forceful, or underhanded conversion of the entire settlement. If that goal is achieved, the whole community would be rebuilt around supporting and spreading the mycus, some of the individuals would go about their business of building a human-looking settlement (possibly with the explicit goal of acting as a trap for humans), some would depart to try and infect other communities, and some might even become members of a fungal/human army.
I think this kind of thing is a lot trickier than you’re even making it out to be. If you’re a stranger and the missionary is settled into the community and not under suspicion already, it would take a lot more than flashing around a badge and making a speech, or presenting a science report stating that they’re infested to get the community to turn on them.
If a community is xenophobic enough to kill one or more of their members on the basis of fungal infection, wouldn’t they also be xenophobic enough to kill mutants on sight?
“Get him! I need to find out what kind of poison that is!”
“But… it’s anti-fungal medicine…”
“You expect us to believe that? Get the bottle, shoot him if you have to!”
Now, you might just assassinate the missionary and move on, but just showing up and killing a community member is going to have a ton of backlash, and you’re going to have a really tough time proving you were acting in good faith unless fungal missionaries were already a really well-known thing.