Slime Tensei: Daikenja ga Youjo Elf ni Dakishimeraretemasu

As his casket was being closed, The Great Sage Marlin Endlight used his infinite wisdom to reincarnate himself into a "Endlessly Evolving Slime". He decides to enjoy his new slimey life, until his third daughter finds him and casts a taming spell on him! Living as a familiar to his foster daughter isn't too bad: Not only is he constantly getting hugs, but he can stealthily continue to evolve and watch over all his beloved daughters. Until he obtains a human form, he will hide his true identity from them. But let it be known that while Sura-chan (ex-Great Sage) is around, no number of greedy fake researchers, bandits or evil cults will harm his girls!