Yes, that could easily be a bacterial bloom.. quite common in any new setup regardless of what you are doing initially with the filtration. The look white/grayish, exactly as if a small quantity of milk were mixed in a large amount of water.
When cloning filters, it is ideal to perform a "fishless cycle qualification" to prove to yourself that the biofilter that has been moved has indeed been a "take" in it's new mechanical home. The vast majority of the time a good batch of very mature biomedia will move right over and be fine, but a small percentage of the time, it will not "take" for one reason or another and something will have to be done about it.
To qualify a filter clone, you simply treat it exactly like a fishless cycle but you fully expect it to pass it's "qualifying week" more or less immediately. To pass a biofilter qualification, the filter needs to drop a dosed 5ppm concentration of household ammonia to zero ppm ammonia and zero ppm nitrite(NO2) within 12 hours of when it was dosed for about 5 to 7 days (basically depending on if cutting it a day or two short allows you the weekend for the big water change and initial fish stocking.)
Note that it also helps at the time of moving the biomedia over, to "clean" the rest of the filter media in the new tank so that still more bacteria (riding on all the messy debris) will be pulled in to the new filter.
~~waterdrop~~
