Both "it totally slipped out of my mind" and "it completely slipped out of my mind" are grammatically correct and commonly used phrases. They are both idiomatic expressions indicating that something was forgotten or not remembered.
Both "totally" and "completely" are adverbs that can be used to emphasize the extent to which something slipped out of one's mind. The choice between the two is a matter of personal preference and style. Both adverbs convey a similar meaning in this context, so either one can be used without any grammatical issues. It's up to you to choose the one that fits best with the tone or style you want to convey.