Stuff ending in "-oid" usually means "shaped like", like "humanoid" or "spheroid", so you can combine it with other words to make a name, like "Arachnoids" for spider-shaped things, which is one option. A number of series mash together scientific-ish Latin or Greek terms like this, like the Xenomorph from Alien (Xeno - alien, Morph - shape/form) or the Biovore in Warhammer (Bio - biological, Vore - to eat).
Some series use the name of the planet they come from, like "Martians" (from mars), "Jovians" (from Jupiter- Jove is another name for the Roman god Jupiter), or the Warhammer "Tyranids" (from
a planet called Tyran).
Finally there's the option that the word is from the alien's own language. Creators often make it sound subconsciously like words from our languages, or have sounds that when spoken sound hard, soft etc. Like the Turians in Mass Effect evoke the word "Centurian", getting across their highly honourable militaristic culture, or the Skrulls from marvel have an intimidating-sounding name, because it sounds a bit like "skull" and it's a word you have to kind of grimace to say.