Looks like it. Did you not notice any cyano during the cycle? If not, were your lights on during the cycle?
I also recently started up a 30 gallon and let it cycle. All parameters were good, but my lights were off during the cycle. During the first week that the lights were on I noticed the cyano start growing on the surface of my rocks. This may be the cause as in your case.
What I did to try and reduce any impact this would have on current livestock:
-Reduce lighting time to about 6 hours
-Weekly water changes of about 30%
-Added some phosphate remover to my filter media
-Try and siphon some off during water changes but do not brush as it will help spread