What do Wrackspurts actually look like? Through Luna Lovegood's Spectrespecs or any spectrespecs, like, are they round or square or transparent or what?
This is what Luna sees in the Half-Blood Prince film. These glowing lights are what she claims are wrackspurts.
Although they're described as "invisible" in the books, this actually ties in quite nicely with Harry description in Deathly Hallows where he describes Luna as trying to beat off midges when she's evading them.
‘That’s her,’ he said, pointing at Luna, who was still dancing alone, waving her arms around her head like someone attempting to beat off midges.
‘Vy is she doing that?’ asked Krum.
‘Probably trying to get rid of a Wrackspurt,’ said Harry, who recognised the symptoms.