Asking Probing Questions During a Usability Study (Janet M. Six)

You are in the midst of a usability study and, when testing one of the tasks, you observe that participants do not seem to see a design element that would help them to finish their task. What do you do now? Do you ask them whether they see the element? Do you just continue and say nothing? Do you ask them later? Is it even possible for the moderator to avoid affecting the test results in some way? (uxmatters.com)