Without lab tests I cannot confirm, but I can state from past experience it seems like mold. The last time I came across that type of mold, it was in a cellar that was exposed to sewage and stayed wet for a few days before being pumped out. I remember we took a tape lift sample from the area. The client stated he tried to clean it with bleach but within a few days it grew back. Some molds in mortar can be tricky to kill.
The words toxic and mold together are mainly used as scare tactics. Again, without testing the type of mold cannot be determined. I'd have to look through my files to see which mold it was, but it may not be the same as your mold.
No matter what you should contact a mold specialist in your area. If you have mold growing, you most likely have spores in the air which also should be killed.
Bleach will not kill the root system of the mold, which is evident by your mold growing back.
If you are in the Boston area, I would be more than happy to help, free of charge.[email protected]