Slime describes any viscuous liquid officially, but can refer to creatures made of gelatinous substance - they're literally called "slimes", but that is not very descriptive and useful.
Goo has a much narrower definition, it describes a semi-solid, generally gelatinous substance, or at most any substance that is similar to hot melting glue.
What if somebody likes the jello-like bits laid behind by slimes or this jello is force-fed to a character then they won't want to see viscous liquids like snail_slime and mucus, and using the blacklist or adding a minus to the query is completely worthless because they have all been aliased to slime.
Incidentally, All these problems together make the way how slime is used bad and forced me to rely on goo_creature for content I'd wanna see.
Although, cum and cum_creature aren't implicated to slime, they probably should be given the broad definition. Slime most likely does get tagged on these occasionally, I can almost guarantee it: slime cum -goo_creature slime cum_creature -goo_creature
Personally speaking, I think slime should instead be aliased to slime_(disambiguation) and then be split into goo and viscous_liquid respectively. The way this works currently is awful.