The “-inp” mode shows the input waveform inverted, and in purple. You can turn it off by going to EX > ext refn > off.
I’d suggest using an LED or some other indication on the 555’s output first, to make sure you actually are getting a signal. You can also test the scope on it’s built in frequency generator.
However, you say you get a single square wave moving along the bottom? It sounds like you are picking up a signal, it’s just not syncing with your trigger. Adjust the y range until the voltage scale matches what you’re expecting, if you have a 5 volt signal, 1 v/div is usually a good choice.
If you get the height of the signal displaying nicely, you can then adjust the trigger level (green line) up or down so it’s in the middle of your signal’s amplitude. Rising or falling edge triggering will determine which side of the input signal is synced with the trigger line.
You also have to play around with the time settings a bit too. Anything too far out of range will cause the signal appear in unexpected ways. For example, a high frequency signal displayed at a large time/div would shows as a flat line, instead of the “thick” band that it should really be displayed as.
Basically, you just have to play around with it (with a known good signal), and you’ll figure it out…