Is it obligatory to remove everything bearing the shape of a cross, out of fear of falling into polytheism (shirk), even if unintended, such as the shape of a "plus" sign (+), and is it obligatory to alert others to this?

It must be noted that not every two intersecting lines are considered a cross, and the plus sign does not fall under the ruling of a cross. Sheikh Ibn 'Uthaymeen, may Allah have mercy on him, states that the true cross is one where there are two lines, one horizontal and the other vertical, with the vertical line being longer on one side than the other. It is necessary to verify that what some people perceive as a cross was indeed placed as a cross, and not merely as an engraving. If it is confirmed to be a cross, then it is obligatory to tear it up or boycott clothes that have it. As for the hexagram (six-pointed star) which is said to be the symbol of the Jews, its ruling is the same as that of the cross. However, if it is apparent that it is not intended to be a cross for veneration or as an emblem, such as some mathematical symbols or a plus sign on electronic watches, then there is no harm in it and it is not considered a cross.