I had something like this in my bucephalandra, it is probably one of two things, firstly it could be what
@ScareCrow mentioned. a genetic mutation brought on by stress. but it also could be a deficiency.
when I moved my buce to my low tech, it started growing these beautiful "variegated" leaves. I had one or two rhizomes of this. I moved it back to my high tech, and it promptly reverted back to normal growth.
once you have a baby plantlet or a runner from this plant, plant it back in the high tech, if it still exhibits the mutation then you;re on to something big.
on a slightly less related note, for a while I was trying to find variegation in some plants. I would cut off a leaf of the plant and let it grow callus tissue, from their some species can regrow a plantlet. the great part? callus tissue has a much higher rate of mutations. so if you have a few thousand leaves and the determination to find variegation youll probably come up with some nice plants. I used to know someone with quite a few plants produced like that. adding co2 from sparkling water can greatly increase the rate of shoots you get, more shoots=more chance of mutations.
rotala species, hygrophila, other stems work fairly well. best to use the weedy stuff.
I've taken a break from such madness, but I'm starting to think about getting the shallow callus ponds out, in hope of producing a variegated rotala macrandra.
this is the closest I've gotten to a variegated plant is this hyperhydrid hygrophila. it is fairly stable and grows very bushy.
sorry for the barely related ramble. I"d been hoping to share this for a while.