It's not clear from what you've written if you applied glue along the joint as well, it sounds like it was just to the biscuits/slots. If so for future reference you need to know the biscuits aren't holding the boards together in a joint like this, they are primarily an alignment aid during assembly.
The glue joint on the board edges provides >95% of the strength in this type of joint (long grain to long grain).
What about wood filler that I sand off?
You can use filler if you like, but a 1/16"-1/8" line of filler won't be very attractive. The main worry would be about structural integrity though, those open joints will have next to no strength and it may cause a problem down the line.
There are superior choices for filler here (epoxy probably being the best and strongest) but since you're willing to saw the panel apart don't worry about that for now.
Or should I slip a hand saw in the seam, cut the biscuits that can be seen...?
That would be the best way to approach this. For someone with a fully equipped shop when something goes wrong with a panel glue-up almost always the advice is to run the problem joint through the table saw and then glue it again.
You can skip adding fresh biscuits this time if you like, but add a couple again if you need the help with alignment during the glue-up. But be sure that you apply plenty of glue to the long-grain edges of the boards this time.
You want to ensure that you apply enough glue that some squeezes out along the entire length of the joint, that's what tells you you applied enough. Clamp firmly as it is high clamp pressure that ensures a good glue bond (don't worry about squeezing out too much glue, this is nearly impossible to do in real-world conditions). You're actually aiming to squeeze the boards together hard enough that the glue joint is nearly invisible, yes, that thin.
In addition to looking better thinner glue lines are stronger than thicker ones with most woodworking adhesives.
Some previous Q&As that might be helpful:
What is a good way to prevent jointed tabletops from bowing when tightening fasteners or the glue sets?
What are the different grain directions, and how do they affect joint strength?
What advantages do F-style clamps offer for gluing up a thick laminate?