When I move a UI.Canvas this symbol appears in it. What does it mean and why is it appearing?

When I move a UI.Canvas this symbol appears in it. What does it mean and why is it appearing?

50865-mystery-symbol.png

Only happens with some canvas component containing prefabs I am using. They are all moved by the same script, there is no discernable difference I can find so far between ones in which it appears and ones in which it doesnt appear. However the prefabs in which it appears and the ones in which it doesnt appear are consistent.

Using Unity 5.1.1f1

Canvases being moved via script.
here is the piece of code in question.

 IEnumerator TurnCoroutine()
		{
			PagesDist = lastpage.transform.position - nextpage.transform.position;
			Vector3 origpos = nextpage.transform.position;
			float nowtime = 0f;
			while (true)
			{
				nowtime += Time.deltaTime;
				if (nowtime >= PanTime)
				{
					BGaccelerator.Accelerate(0f);
					nextpage.transform.position = destination;
					lastpage.transform.position = nextpage.transform.position + PagesDist;
					Finished();
					break;
				}
				else 
				{
					float ratio = Curve.Evaluate(nowtime / PanTime);
					BGaccelerator.Accelerate(ratio);
					nextpage.transform.position = Vector3.Lerp(origpos, destination,ratio);
					lastpage.transform.position = nextpage.transform.position + PagesDist;
				}               
				yield return null;
			}
		}

Which version of Unity are you using? Are you moving the Canvas with a script or with the Scene transform tools?