Hi. I wanted to know if it is safe to use TDA2030A 18W Digital Audio Amplifier Board DC 5-12V Mono Single Channel with 9 w speaker in arduino
Yes.
You'll probably be OK, but It depends...
Just because you have a 18W amp, that doesn't mean you have to run it a full-power. 
Power is calculated as Current x Voltage. If you are running 18W into a 4-Ohm speaker and you replace the speaker with a 8-Ohm speaker, you'll get half the current (9W).
Or, when you double the voltage you also double the current. Power can also be calculated as V2/R. That means if you are getting 18W out of an amp running from a 12V power supply and you switch to a 6V power supply, you'll only get about 4.5W.
Speaker power ratings are for undistorted program material, not continuous tones. That is, a 9W speaker can handle a 9W amplifier that's hitting 9W on the peaks. Music or voice hitting 9W on the peaks will be running at about 0.9W RMS (or average) depending on the type of program material. A 9W speaker will handle 9W of undistorted voice/music, but only about 1W continuously... You'll fry it with continuous 9W test-tones, or if you use it as a siren, etc. And, that assumes that speaker ratings are honest, and often they are "optimistic."
Anvay:
TDA2030A 18W Digital Audio Amplifier Board
Sound like a corrupt ebay seller statement.
The TDA2030 is an ancient analogue audio chip, commonly used in 5watt guitar practice amps.
Expect no more than 4watt RMS with a 4ohm speaker on a 12volt supply.
And 2watt RMS with an 8ohm speaker.
Leo..