After i construct dynamic tank i have inside Mob Essence.

• Versions:
  Mekanism: 7.0.0.118 ,
  Forge: 1208 ,
  MFR: 2.8.0RC3-591

)

Most likely because one of the tank blocks that formed the structure once contained mob essence. No other way this could happen, but it is strange for sure!

Yes this is strange if a break all blocks and i again construc dynamic tank mob essence appear.