Okay so hear me out. Alien who is very close with their human crewmate, and is used to you smelling like the various products humans use for personal hygiene. This is perfectly fine, maybe they even grow an attachment to your preferred brands of deodorant and lotion, but it leaves them completely unprepared the first time they smell your unmasked body odor and realize oh yeah, humans are very sexual creatures and produce pheromones accordingly. It's the alien equivalent of seeing someone you already like half-naked and having an "Oh no they're hot" moment, except you don't even realize the effect you're having on them because you just got done training or woke up from a very sweaty nap and just want to shower.

Smiling is pretty unique to humans on Earth, so do the bug aliens have anything equivalent or will they always think the expression is kind of weird? Also what do they call their own species because I figure it's not big aliens (acknowledging that they probably have at least a couple different languages like humans themselves)? Actually now that I think about it, how the fuck does Rhaxa know how to speak English (or what language they're speaking because this is a constructed world)?

i think they'll likely always have a deep-seated instinctive reaction to smiling, but also they are a very body language-focused species so they're very adept at learning people's mannerisms quickly, even other species - and being around humans for an extended period of time would likely result in them acting more 'human' in a lot of ways, sort of stooping to our level in order to better communicate.

their own name for their species isn't something we'd be able to pronounce - again, they rely heavily on body language and pheromones to communicate with each other and express ideas, so rather than have a specific word it's more of a Vibe that they express through certain behaviours. they probably don't have hundreds of distinct languages in the way humans do (worth noting here that they are also significantly less populous than humans, as well as having a more fluid, less geographically segregated population), but groups will form their own dialects and become better at interpreting intent from individuals they're more familiar with. there can often be some teething problems when new groups encounter one another, with the time it takes to figure out communication strategies.

rhaxa and imxa know how to speak english because they have been trained or have experience with humans; not every alien would be able to speak to humans, or to understand them. and also from a biological perspective they just have the vocal cords and laryngeal structure necessary because i said so - irl any of the species i use as inspiration for them would have completely unsuitable vocal structures to create the necessary sounds for speech. i'm kind of accounting for this by saying it's not Comfortable for them and they prefer not to talk out loud for long periods, but there are times i'll have to ask yall to suspend your disbelief, as a personal favour to me.