Divi accordion plus (+) icon goes out when you click an accordion title.
If you want to remain there and be changed to minus (-), add these CSS lines to style.css or on the same Page > Advanced > Custom CSS
.et_pb_accordion .et_pb_toggle_open .et_pb_toggle_title:before {
display: block;
}
.et_pb_accordion .et_pb_toggle_open .et_pb_toggle_title:before {
content: "b" !important;
}