Your tank is rather young for clams, but that's not to say you can't keep them, it's just going to take some work and diligence on your part. Also, keep an eye on the flame angel, they can nip at can mantles. Diatoms are usually a sign of a new tank and they tend to disappear on their own as the tank matures. CUC can be handy during these times. Ceriths and aestrea snails do a good job. I would get a par meter or rent one, it will really help in placement of your corals and the clam. Tridacna clams need light, they don't need to be fed, but they do need the proper amount of light at the right spectrum.
As far as your original question regarding the intake siphon, it's normal for a clam to have a wider opening, especially when introduced to a new tank or system. It usually goes back to normal as soon as it settles in and your parameters are okay. If it stays open, it could be stressed and it's referred as gaping. I would dpi a lot of reading about clams, if you haven't done so already? Daniel Knop, James Fatheree are the go to guys when it comes to clams.