Update: If the Invisible Ink option does not appear in your Messages app, it is because you have the Reduce Motion slider enabled. Navigate to Settings -> General -> Accessibility -> Reduce Motion, then tap the slider so it is OFF/WHITE.
Update 2: You can only send message effects to someone who has an iOS device running iOS 10 or later. If you have a green arrow instead of a blue arrow this either means the person you're messaging does not have an iOS device, or you have iMessage disabled. You can enable iMessage by navigating to Settings > Messages. Tap the iMessage slider so it is ON/GREEN.
Update 3: iOS 10.1 allows screen effects to be played with Reduce Motion enabled.Apple added several new messaging features in iOS 10, such as the ability to send Invisible Ink or animated effects in the Messages app. The Invisible Ink feature allows you to hide your text behind blurry animated pixels. The person receiving the text can read your message by wiping away the invisible ink with their finger to reveal the hidden text. Here's how to send an Invisible Ink text message in iOS 10 or later.
- Type the message you want to send like you normally would in the Messages app.
- Tap and hold the blue arrow in the text field until the "Send with effect" screen opens.
- Select the INVISIBLE INK option then tap the blue arrow to send the message.