A typical 1st level character has an attack bonus of +5, which means they will probably (55%) hit AC 14 on the first attempt, doing an average 7.5 points of damage (longsword), killing it outright. Bludgeoning damage isn't particularly common in 5e, since wizards are more likely to use firebolt than hit it with a staff, so it's unlikely the party will even notice it's resistance. If you intend to try and overwhelm the party with numbers, I would expect a typical party of four 1st level characters to kill at least two of these per round.

Mechanics aside, part of designing a good monster is that the players should be encouraged to suspend disbelief by the creature's stat block matching what they intuitively expect. Now, you can set out to challenge expectations, particularly if your players are experienced, but in that case it is best to only have one thing that is odd about your creature. Your creature is odd (for a slime) in FOUR ways - it has the wrong resistance, it has armour, it is fast and it can dissolve armour.

I would change it in the following ways: it is resistant to slashing and piecing damage as well as bludgeoning, it has no natural armour but it's dexterity is 18 (this will give it a bigger bonus to initiative, increasing the chance it will get an attack before it dies), it has a movement speed of 40' - if you want to make the point that it is fast it should be faster than the PCs. That would give it a CR of 1/2, but I wouldn't worry about it - a first level party should still win against three or four of these, as their damage is quite light.

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