Spilled coffee on my pants.Took them off to dry. Suddenly thought of Marie. Remembered sliding into her cute pussy. Underwear off, cock in hand. Try to resist but her pussy felt so tight and soaking wet. Hand sliding now, cry out her name, "Marie! Marie!! Marie!!!" louder and louder. Try to stop but can't. I can't stop jacking off to Marie. I remember straddling her chest, staring at her tits as the cum squirts uncontrollably all over. My balls tighten and now I scream, "Marie!!!" as the cum squirts all over the mattress. I lay back and imagine her smile. My never stops sliding on my cum-soaked cock. My phone beeps. We meet tomorrow. I look at the cum everywhere. Should I tell her what she is doing to me?

Well if your actually riding eachother than it mightn't be any harm but if she thinks it's more than a bit of adult fun maybe it's better not to.
If shes just an acquaintance you shouldn't even have to ask this question you'd be lucky not to end up in jail and I wouldn't blame her one bit. By all means enjoy your private fantasy but no stalking.

Not sure what you are getting at here.

We're you reliving something that actually occurred between the 2 of you, or is this all a fantasy?

If the former and you just had a great stroke session to a shared memory then yes, you should tell her.

If this is all in your head and she was never part of your scene IRL, then it is a crap shoot, 10% chance she will think it hot, 20% chance she will be neutral and 70% chance she is creeped out and will avoid all contact with you going forward

Marie is my lover.