I have Apple Watch series 7 and just got new iPhone SE (I had iPhone series 7) can I use the ECG app on my current SEC phone

Yes, if you meet all of the requirements for ECG.

Here's what you need

• Make sure that the ECG app is available in your country or region. Learn where the ECG app is available.
• Update your iPhone to the latest version of iOS and Apple Watch to the latest version of watchOS. 
• The ECG app is not intended for use by people under 22 years old.

You can use the ECG with your Series 7 and the iPhone 7 you had. The SE will also work

According to the 'Here's what you need' section in:

Take an ECG with the ECG app on Apple Watch - Apple Support

as long as the series 7 watch pairs with the SE iPhone, it should work.