Stacy didn't show her mom on purpose. She was sitting down with her back to her mom. Not really thinking, she pushed her hair aside and that's when her mom saw the bruise.
No, I got the distinct impression Stacy showed it on purpose. Watch her facial expression as she does it, making sure her mom is facing her neck.... and keeps her hair that way till her mom sees it.
Then when her mom asks her what happened, she stutters a bit & then lies. She wanted her mom to sit down & dig deeper, which she did.
Stacy was likely about to tell her the truth when Rod showed up, and Stacy chickened out yet again.
I'd say this cloud is Cumulo Nimbus. Didn't he discover America? Penfold, shush.