Saving Bark Tanning Liquor

I'm just getting into bark tanning and I've brewed up a batch of bark liquor using red alder. I'm storing it in buckets and they all look great but one bucket is getting slime on the top. Is this an issue? Should I be adding salt or something to stop it? Was it just that this particular bucket wasn't washed properly? Any insight would be helpful.

Not sure why one bucket would and the other wouldn't if they were made from the same shredded red alder and stored under the same conditions. I'm sure somebody with more knowledge than me on the topic will come along and be able to help.