When you run the design rule checks, "DRC error/violation markers" are added to the PCB view, showing where objects violated one or more design rules.
The simplest way to clear them is to press T then M, which is the same as accessing the Tools menu, then selecting Reset Error Markers.
In your specific case, the lines that sort of look like a capacitor are shown for design rules under "un-routed nets:"
If (for some reason) you just wanted to get rid of those particular error markers, you could disable this rule and re-run design rule checks.
As others indicated, there were/are bugs in Altium which sometimes leave the error markers shown despite making changes to the PCB.
You can also simply hide them by using the System Colors section of the View Configuration panel, though I don't recommend doing this because it's possible to forget you hid them and spend precious time investigating why DRC errors aren't showing up. (This may or may not be based on experience.)