Flashing is when your fish will rub its body on a rock due to irritation from something. If its doing that then you might want to consider he may have a parasite. Any new changes ? Adding a new fish , rock or coral. I'm currently having this flashing problem with my coral beauty angel.
I'm in the same boat as you. Lol try moving some live rock near the filter and keep a eye on him. Lastly check your ammonia and try to see if anyone is picking on him. What type of live stock do you have.?