Hi nincolors, welcome to fishlore.
What kind of veggie is it? My mollies pick at everything, so long as I remove any tough skin (like on cucumbers or zucchini). If it's lettuce or something like that, then just hold a corner down under a rock or use a veggie clip, then they'll pick at it all they want. Remove it after a day or so because it gets kinda gross.
If you wanted, you could toss it in boiling water for a few second, then drop it in ice water to blanch it. That will make it a little softer and easier for the fish to eat.
What color are the strings?
That's odd behavior (the vertical hanging) directly after a
water change. Was the water a vastly different temp? Perhaps a
pH shock? Did you remember to add the water conditioner? Is the tank already cycled?