Speaking to Soaplife, the actress revealed that the pair's fling will last until Moira's son Adam returns to the village from holiday.
The star disclosed: "Once Adam is back, Alex's attitude changes a bit. He begins to see the consequences of it while Moira still thinks it's just a bit of fun. She doesn't think anyone will ever find out."
Clarifying the terms of Moira and Alex's relationship, Robb said: "In no way at all is Alex a replacement for John. In a way, the fact he's so much younger and it can't possibly go anywhere is the reason she's able to do it. It's fun and the sex is good."
The 37-year-old recently commented on her on-screen chemistry with 21-year-old co-star Kurtis Stacey, who plays Alex.
Emmerdale viewers will see Moira and Alex finally succumb to their passion on Friday (August 10).