Taste it. If it's not too objectionable, keg it and drink it... fast. Otherwise dump it. Or let it ride for a while and see what you get. Might be a nice sour or it might be a batch of vinegar or it might be something else altogether.
As for advice, there's too much headspace in that bucket, which is likely why it's infected. In fact, buckets are terrible as a "secondary" due to the fact that it's pretty much impossible to not have too much headspace. They are great for primary (I use them myself), but should never be used as a clearing or aging vessel. Use a carboy instead and fill it all the way to the neck. Or don't bother with a secondary, just keg it when it's at FG and call it good. If it's a recipe that needs to age, age it in the keg as long as needed.