You say that you had it under control and the only thing different is your light so you may want to think about either raising your light, cutting back on the photoperiod or putting some screening between your light and tank until everything is used to the light. Now that the cyno has gotten a foothold you may want to think about cleaning your tank, sand and rock by taking the sand and rock out and rinsing it really well in some fresh saltwater and doing the same with the sand. It is obviously finding something to feed on in your tank so maybe cut back on both feeding and light period for a while.