when i was exploring my world it was announced that mob captain just spawned, i saw purple particles just like chat message but there was no mob. The only other mob mod i have installed is mobs improve over time

What Minecraft version are you in?

Other mods may very well mess with it but if the message appeared and the particles appeared, the mob very likely did spawn - maybe you didn’t find it because it was underground? Or it despawned before you got there? (That last thing is only possible in older versions)