You should move this outside of the PromptShown scope, because this causes your code to connect a new function to the PromptHidden event every single time a prompt is shown instead of once which could cause a memory leak
ProximityPromptService.PromptShown:Connect(function(ProximityPrompt)
local Highlight = Instance.new("Highlight", ProximityPrompt.Parent); Highlight.FillColor = Color3.fromRGB(255, 255, 255); Highlight.FillTransparency = 1
end)
ProximityPromptService.PromptHidden:Connect(function(ProximityPrompt)
local Highlight = ProximityPrompt:FindFirstChildOfClass("Highlight")
Highlight:Destroy()
end)