Use display: flex property and it make easy to align the content.
Along with it use align-items: center; to vertically center align
.improvement_request_current {
  display: block;
  background: #777;
  display:flex;
  align-items:center;
}
.improvement_request_current_title {
  background-color: #999;
  color: blue;
}
.improvement_request_current_value {
  display: inline-block;
}
<div class="improvement_request_current">
  <span class="improvement_request_current_title">Just some text</span>
  <textarea class="improvement_request_current_value" rows="6" cols="50" maxlength="4096"></textarea>
</div>
 
 
Also one other method can be providing fixed height to parent and child(here given 100px) and than vertical-align: middle;.
But I don't recommend this method which you can see by removing comments from the background property and see that it is spanning out of parent element
.improvement_request_current {
  display: block;
  background: #777;
}
.improvement_request_current_title {
  /*background-color: #999;*/
  color: blue;
  display: inline-block;
  vertical-align: middle;
}
.improvement_request_current_value {
  display: inline-block;
  vertical-align: middle
}
<div class="improvement_request_current">
  <span class="improvement_request_current_title">Just some text</span>
  <textarea class="improvement_request_current_value" rows="6" cols="50" maxlength="4096"></textarea>
</div>
 
 
Temani Afif has given a short and simple solution, using vertical-align: middle to text-area will solve your problem
.improvement_request_current {
  display: block;
  background: #777;
}
.improvement_request_current_title {
  background-color: #999;
  color: blue;
}
.improvement_request_current_value {
  display: inline-block;
  vertical-align: middle
}
<div class="improvement_request_current">
  <span class="improvement_request_current_title">Just some text</span>
  <textarea class="improvement_request_current_value" rows="6" cols="50" maxlength="4096"></textarea>
</div>