Ixora coccinea is native to tropical Southeast Asia, including India and Sri Lanka, and is therefore out of our range of expertise. The Lady Bird Johnson Wildflower Center is dedicated to the growth, propagation and protection of plants native not only to North America but to the area in which those plants are being grown. This Floridata article will give you more information, but does not mention a disease such as you have described.
This sounds like a sooty mold, which can often be caused by mold growing on the honeydew exuded by aphids. However, we are not sure if that mold would spread to a wall. This article Floridaplants.com has information on pests and diseases of Ixora coccinea. Of special interest in your situation is this excerpt:
"Insects and Diseases - This plant is subject to nematodes and should be mulched heavily. It is also attacked by aphids, mites and scale, which cause sooty mold."
If the plant, and especially the wall behind it, are not getting enough sun, the possibility of a mold gets higher. We suggest you examine the plant for aphids, and go from there. Please read this previous Mr. Smarty Plants answer which points out that the honeydew and the resultant sooty mold can spread to surfaces around the infested plant.