If it was stuck to a plant stem...about 1mm in length......it would be a scale insect. Look for a spray to control it. The scale insect will suck the sap from your plants and weaken them. If it is on a house plant, you can scratch them then wash them off.

Yep, scale is what it is - I think Provado have something that might treat for this, but if there aren't many, a cotton bud dipped in methylated spirits and applied to the scale insect should enable you to remove it more easily, and will kill it anyway.