An HSA is like any other investment account - it is simply a basket and it is up to you to figure out what to put in there. You've got money in there but as others have pointed out, you have to invest it or it'll sit in cash.
If you are actively using the HSA to reimburse for medical expenses, you should leave some portion in cash. That way you don't have to sell every time you want to reimburse yourself. While that's not the end of the world and there would be no tax consequences for having it all invested and selling investments every time you want to withdraw, I find it to be an annoyance to deal with the time it takes to have the funds settle and then to transfer to my bank.
Remember, if you do not need to reimburse yourself for medical expenses, you are under no obligation to do so. Keep track of your medical expenses and keep your receipts. You can reimburse yourself for your medical expenses at any time, even if it's in 20 years. And in the meantime, your money can go in pre-tax and grow tax-free. And then when you want to withdraw, you can do so tax-free up to the amount you've spent on medical care.
To put some numbers to it, let's say you contribute $3400/yr ($283.33/mo) for the next 20 years. That's $68k. Let's say that in that 20 year period, you also had medical expenses of $100k but you never reimbursed yourself from your HSA. Over the 20 year period, and assuming a 7% return, your contributions will grow to approximately $143k. At the end of 20 years, you can withdraw $100k to reimburse yourself for medical expenses and every penny would be tax free. And you'd still have $43k left.
