Spider mites are having a heyday eating the new growth so the plant shut down. I use the garden hose to spray off any bugs and eggs then treat with a neem, spinosad, or 3 in 1 product

It looks like it may be a seedling?

It's 3 years old. How can you tell it is spider mites? Is it because of the webs? I am not sure I have spider mites this year on my plants, but I sure have a lot of tiny spiders. Can they cause this? I thought they were harmless.

This plant is about 60 cm tall. I have two others at the same age and I think are doing ok. I don't know what went wrong with this one. If it's the spiders it must be really sensitive because the spiders went also at the others.

To an older plant which has more leaves they are as with many things in small quantities spider mites are generally harmless. Some plants for whatever reason may not be impacted as much as others. I have had Plumeria next to each other and one is infested with spider mites and the other has nothing.

You can see in your picture just above the one current leaf the spider mite web, a light color speck, and the look of the damage done is telltale. I hope that helps you.