Cyano algae/bacteria....
As timbo indicated, it feeds off P04 - also nitrates.
Loves low flow/current areas - try increasing water movement in the problem areas.
Old lights? Aging bulbs shift spectrum, and this is often offered as a cause of
cyano outbreaks
Cyano needs food & light to thrive (as do most algaes). Food comes in thr source of phosphates, nitrates, etc. Good nutrient export (limiting feedings, using
RO/
DI water, possibly using a refugium, good skimming) - all these reduce the nutrients in your system. Also, maintain a low bioloads, having sufficient
LR, and a
DSB are quite helpful.
Watch the age of your lights, maker sure water flow/current is good, and keep a decent cleaner crew -- these are all excellent preventative measures.
Many ppl resort to antibiotics ("red slime removers") to battle
cyano. This
can be successfull if you've eliminated the orignal cause of the
cyano, and are trying to "clean up" the remnants. However, if you have not solved what is causing the
cyano, and you use an antibiotic, it is almost certain to return.
Good luck!