When I went to write a text message today I discovered a voice message icon in the text box. Why is this and how do I delete it

A significant number of people are having that problem since iOS 9. One solution (at least a temporary one) is to quit Messages and then double-click the Home button to open the app switcher. In the app switcher find the Messages app and slide it up to remove it. Then close app switcher by clicking the Home button. That should solve the problem -- for a while at least. Another solution that is working for some is to go to Settings > Messages and toggle either the Show Subject Field or Character Count setting.