@Gakamis
Usually it's as you say, but Slime is quite special, you see. The Slime manga is infact the best-selling manga adaptation of a Light Novel to have ever existed and I am not exaggerating here. Add to that the fact that the LN publisher of Slime never had any of their works be turned into an anime yet, especially not on their own (the series that will have that role will be
Kenja no Deshi instead), so to make sure that such a gold mine of success as Slime does not get butchered, the manga publisher Kodansha took it upon themselves to produce and finance the Slime anime instead, though they got helped by Bandai among whom a big-shot really enjoys the manga and hence wanted to see it animated himself.
Anyways, it's a fact that the manga is the main "source material" of the anime. If you look at the Slime Anime homepage's staff section on https://www.ten-sura.com/anime/tensura/staffcast then you'll see 川上泰樹 (Taiki Kawakmi) as the first name under "original creators", which is the manga artist, followed by 伏瀬 (Fuse) the author and みっつばー (Mitz Vah) the LN illustrator and character designer, and underneath those three names it says 『転生したらスライムだった件』(講談社「月刊少年シリウス」連載) which means "Tensei Shitara Slime Datta Ken (serialized in Kodansha's 'Monthly Shounen Sirius')" which is where the monthly manga chapter RAWs, that we scanlate for you here, come from. If the source material was the LN, it'd leave out the manga artist or put him at third place of the creators, and it'd also say "serialized by Micromagazine" or "serialized under the GC Novels label" instead.