Yes, that layer is your viable yeast. Decant the top off, pour that in to another jar and don't worry if a little trub follows. You have 3-4 weeks to make another beer until you should probably make a starter. I sometimes go up to 2 months depending how good of a separation and wash I got.
How long the slurry is 'good' without a starter is based on volume and viability. You can wait 3-4 weeks as long as you have enough slurry.
I usually take the Mr. Malty calcs as gospel when it comes to slurry age/viability, even if they are a bit conservative.
Personally, if its been longer than a week, I'll make a small starter to boost viability and reduce lag/cleanup time, especially if its a finicky yeast or a lightly flavored beer (Kolsch, helles, Pils, etc).