Sounds like you have a cyano
problem. It can smother corals given the chance. If this is an established tank, the main reason will be from PO4
as a rule. Check the levels in the tank and from your source water to determine the PO4
level. Any amount will fuel cyano
once in the tank.
Given this is a 40 gal
tank, it is also overstocked:
2 clowns, 1 regal tang, 1 yellowtang
That will also be a contributor. Even one tang is too much for this size tank.
Reduced feedings, increased water flow, numerous water changes to reduce DOC
's and other possible polutants as well as manual syphoning is the best method to control/eliminate cyano