What does it mean when your current job opening says grade E-5, but rank is SPC?

It means you're working one grade up. This is one of three things: (1) you're being given a pat on the back, (2) your COC is demonstrating their faith in your abilities, or (3) they haven't got an E-5 to fill the slot and as soon as they do you're going to be bumped back down. Number 3 is most likely.
In any case, it's no biggie. Since you won't be getting an NCOER this will not reflect later, unless they give you some form of official recognition. This is completely normal. The army has a two-up, one-down policy which means that, in the absence of a soldier of suitable rank, the CO can slot someone in up to two grades above or one grade below his or her actual rank. It has no effect on pay, date of rank or seniority, unless you are slotted in a supervisory position - in which case, you may have the ability to give orders. But that will revert as soon as someone who outranks you joins the unit.
Still, it gives you some bragging rights with the other E-4s in your unit... but be careful. They might think it makes you liable to buy the next round.
Congrats, sort of.

If a position on the TOE or TDA specifies a requirement for the rank of Sergeant (E-5), but also allows for it to be filled by a Specialist (E-4), it means that either a Sergeant or a Specialist may be assigned to that position. These personnel manning documents will often state a grade "requirement" for a position and what grade is "authorized"; which is sometimes a lower grade authorized. If you're selected to work in a position with a grade requirement higher than your grade then consider it a nod that the Army sees your potential.