Looks great! Do we want to modify the sporocarp at all, or leave it as is? I guess we could reduce the damage from the spore infestation.
Didn't the description suggests the silver slime's spore burst was more a minor nuisance than a life-threatening hazard?
I believe so.
Why not just 2d4 acid damage which doesn't continue or need to be scraped off? And no more spore infestation. Like this:
Yeah, CR 1/2 is enough.
Anything mechanical left?
Tactics:
In swarm form, silver slime will first try to slather as many opponents as possible in its concrete and then coalesce into a slug.

(Dungeons & Dragons)
Rulebook featuring "high magic" options, including a host of new spells.