Upset and in a custody thingy (would say battle but we're not too sure)?

i'm the father of three kids one of which isnt mine biologically. my stepdaughter is going on six years of age and i have been there for her every step of the way. just recently, her biological "sperm doner" has tried to step back in her life. he is not on the birth certificate or anything but he has had a blood test done. just recently he got upset and started posting, on facebook, that he had a lawyer and trying to say that i smoke pot and drink around her and that i beat her of all things. that doesnt too much worry me because i can disprove all of this, but i did want to know if that is considered slander and if we can use this in court. now, he is saying that within two months he will have custody of my daughter, full custody. and he is just bragging all over facebook. we are very good parents that have always provided for our kids and would not harm them whatsoever. do i have anything to worry about from this guy or is it just facebook drama? God knows that i want to beat his face in but i cant because that would cause more complication than good. someone please help me on this subject. i really need to know how the law works on these types of things. (one more thing, this guy has a criminal record and got my wife pregnant at the age of 15 when he was 20, which worries us when it comes to our daughter.) thanks in advance guys.