How much net carbon can coral sequester?
Corals usually grow 1cm every year, some as slow as 2mm, some faster. 

Coral sand density is about 1.5g /cm2, it's not the best estimate of coral weight. So, a coral organism of 10cm^2 produces 150 grams of CaCO3 every year, and 1m2 of coral can absorb 15 kilos.

CO2 is 44.01 g/mol 
CaCO3 is 100.0869 g/mol

15kg * .44 is the absorbtion of CO2 of a coral organism of 1m2 surface area. 
that's 6.6kg. 

A car produces about 6 tons of CO2 every year. 

Coral reefs are estimated to cover 284,300 km2 (109,800 sq mi), just under 0.1% of the oceans' surface area.

There are 1 billion cars, and 9 billion tons of antropogenic CO2.