Wow, you read my mind~ I was going to post something about this today since I had problems with a large batch of the mousseline (for 150 in the wedding cakes chapter) this weekend.
My advice is to scrap it and start from scratch.
I had a soupy batch of buttercream (my only guess is that the eggs weren’t cool enough) flavored with lemon curd that I tried to save (and ended up using to crumb coat the cake) but I should have just bit the bullet and thrown it away immediately.
If it doesn’t set up properlly to begin with, I would set it aside and start a new batch. I made two smaller batches of mousseline that I made after my precious* first batch remained soupy and they both turned out perfectly.
Don’t do what I did ~ try to salvage the buttercream ~ just start again. The crumb coat (which was the soupy mousseline) started to weep through the outer perfect frosting and created not only a marbled appearance but was far too tempermental to sit outside for hours. I ended up needing to chill it at the reception site and hang around to serve the finieshed cake at the weeding.
That was nervewracking and I wish I had just thrown the first batch away! Believe me, I know how painful it can be to throw away all those ingredients.
*Precious and painful because I had made the lemon curd from scratch to flavor it!!!!