I want to install a 120V outlet for my deck, and I'm not sure if I can run this from the same load center that my hot tub uses. It's a 240V load center that is fed from a double pole breaker in the main panel inside. If I can, I would much rather run the outlet from this panel instead of running it from the one in my garage and digging a trench to the deck from there.
Get that bond screw pulled, and get a PK7GTA in there, then you can add a circuit here
The first step is to sort out this morass of a spa panel box. The good news is that it's a QO612L100RB, which means that you can get just about any sort of breaker you'd ever want to put in there. The bad news is that the original installer butchered the neutral/ground handling, leaving the bond screw in and spamming all the ground and neutral wires onto one bar, so we have to fix that before we can start adding circuits.
You'll need a PK7GTA ground bar and a screwdriver to fix this, in addition to the breaker for the deck outlet. First, turn off power at the feeder breaker for this panel. Second, you'll need to remove the green screw next to the neutral lug; this removes the bonding screw from the panel, and separates ground from neutral here. Third, you'll need to install the PK7GTA in the panel, screwing it to the raised holes bottom left or bottom right. Finally, you'll need to take the two bare ground wires out of the neutral bar and connect them to the new ground bar instead.
Once this is done, you can add your branch circuit as normal. The decision as to whether to use a GFCI receptacle with a regular breaker or a GFCI breaker with regular receptacles is yours; the NEMA 3R enclosure of the panel might provide marginally better protection for a GFCI than a receptacle box would, but GFCI breakers are much more expensive than even a weather-resistant GFCI receptacle, so there is that as well. Don't forget to size the breaker appropriately for the wiring you are running for the new circuit (15A for 14AWG, 20A for 12AWG), use properly outdoor-rated wire (UF instead of NM, or THWN in conduit if you are going the conduit route) and boxes (with while-in-use covers), and torque all breaker and loadcenter screws/lugs to specification with an inch-pound torque screwdriver (as per 2017 NEC 110.14(D))!
Great - looks like you have 4 double (240V) spaces, with 3 of those free, so you have space for as many as 6 single (120V) breakers.
How much spare capacity you have, if any, will depend on the feed breaker in the main panel, and the actual load on your 50A hot tub breaker.
That is, if the feed is also a 50A breaker, your 120V load would have to be less than the difference between 50A and the actual load of the hot tub, or you risk having the feed breaker trip. Please note that you don't need (say) a 70A feed to have a 50A and a 20A at the sub-panel - but if they were fully loaded, you would. Note that (were that the case) you could have 2 20A (120V) breakers without needing any more than 70A maximum feed, so long as they were on opposite legs (generally for this type of panel, side by side) since the 20A on one "leg" balances the 20A on the other "leg", so 50 + 20 + 20 only comes to 70 if the 20's are properly arranged 120V 20s.
If your anticipated deck outlet loads are small, you probably have nothing to worry about. If they are larger, you might need to upgrade the feed breaker and perhaps also the wire from the feed breaker to the sub-panel.
You may also have a problem in that I think it looks like you may NOT have ground and neutral isolated in this subpanel, which they definitely should be.