We pretty much know which "agents" Moghedien is talking about, as we know when she discovered Nynaeve's existence.

She must have asked someone in Liandrin's group (and that this is the explanation is supported by the fact Moghedien's "agents" aren't in the Tower where they'd have spotted Sharina for instance). Liandrin's group mustb told her about the Tower's strongest women, it's even likely it happened after Moghedien saw Nynaeve's strength for herself (it's her, not the BA who found Nynaeve in Tanchico), so much above those of those BA. That must be what sparked her curiosity - for her own safety she had to know if she'd encounter many more like Nynaeve. She must have asked the BA if there were other AS like Nyaneve or stronger.

Basically, that quote just tells us Moghedien found Hessalam to be of significant strength. It also pretty much tells us Graendal was strong, but not in the "freak" zone like Lanfear because Moghedien wasn't surprised enough for that, nor did she think of Graendal's strength.

IMO it tells us Graendal is either in Nynaeve's range or somewhat stronger, but still in the zone of strength that's rare but not exceptional enough for her to think "wow, this woman is exceptionally strong, about as strong as Graendal". If Moghedien met someone else of Lanfear's strength, it's something like that she would think.