How can I turn off the memory limit is too low message?

If your memory limit is lower then 60M, then you will receive this error message in the frontend:
“It seems like the memory_limit on the server is too low for the background resize. Please set it minimum 60M and reload the page! You can disable this message in global configuration ‘Frontend debug messages’ option.”
This is more of a warning, then an error message, so if your sliders show up in the frontend, and the Smart Slider is working in the backend, then you can just turn this option off at the Smart Slider’s SettingsFrontend debug messages, and after you done this, or if you have increased the memory limit, you will have to refresh the cache at your slider, where that message is, so it would disappear.