Since it's a 555 timer chip, then pin 5 is a control input, but in your case it looks like it's not used; so normally one would use a cap from pin 5 to ground, the -Neg supply rail, to reduce any risk of noise pick-up into pin 5 control.
To answer your question; the cap shown is polarised, being a "3.3Mfd or uf" and it might be required ? if, the 555 is driving an output power device that's creating a lot of noise within the circuits DC supply rails... "if that's not the case" and your 555 is on it's own, just doing some simple low power output work, then any non-polarised cap within the value range of 10 to 100nf, would do the job.