3 days with no light will kill it, but has shown some undesirable affects with acros and montis, often associated with a pH drop.
Treating with antibiotics like the I clean can work, but if any of the bacteria survive the next round will be worse, so on and so forth.
Cyano is a very efficient reproducer. It's able to round it's structure every 20 minutes. So leaving just a little bit will turn into a lot a bit quickly.
IMO find the source of the problem and deal with that. Then if it were me and I didn't have all the sticks I have I'd do the 3 days with no light, and monitor the tanks pH as best you can.
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