can you tell how to play softer without all the air and hissing sound and still have a good tone for alto

yeah check your sax maybe theres something wrong with it.

If it's not a leak, get the reed you usually use and get sand paper. Rub it against the reed and try it on every once in a while. Then when you think it'll work best, smooth it out with the other side of the sand paper. It'll help air get in more easily and without any blackages so no spit will get stuck there. Spit is what usually makes the hissing.

It might be your ligature.

To see if it is, put your reed on your mouthpiece but don't put the lig on. Hold the reed tightly with your thumb. If it doesn't make an airy sound, then it's your lig.

Usually its not the saxophone, it just might be your embouchure and use of air. When playing loud, you increase the air speed and open up the oral cavity more. In contrast, when you play softer, you should blow the air softer, and MAKE SURE your embouchure remains the same throughout the range of your instrument, most people have a misconception that you "tighten up" when playing higher or softer. This is not the case! As for the airy sound, make sure the corners of your lips are completely closed so no air is escaping. Also remember MALT(more air, less tongue). Too much tongue can cause that airy, sometimes slappy sound. Hope I helped!!!