Can you post a photo? Your description doesn't help much.

I have 5 Barbados Cherry plants, and none have had any pest or disease issues. Is your tree potted, or in the ground? How old? Is this a brand new problem? What fertilizer do you use? Has your plant bloomed and fruited?

"Black powdery thing" might be sooty mold.

Safe treatments for unknown pests would be insecticidal soap sprays (Home Depot has it) or Ultrafine oil, also at HD. Neither should hurt your tree, and they will take care of most pests.

Do you see insects? How do the leaves look? A healthy green or dry, mottled or falling off?

Lisa

I had Cottony Cushion scale and then the black sooty mold showed up as well. I sprayed a couple of times with Safer Insecticidal Soap. This help a lot and then I took the hose and sprayed/wiped off the remaining scale. I did this a month ago and the tree still looks clean and growing well.

DW

I will try to post a photo tomorrow because its too dark to take a photo right now. But i did google that "sooty mold" you mentioned and it does seem like it is that.
But to answer all the questions.. the tree is in the ground, i only planted it this year around march but its like 3-4 feet tall. There is a lot of ants that live at the bottom of the tree. And i used to see aphids on it. The problem seems to be pretty new, i only noticed it recently.