My husband pointed out some weird night time creatures we have. They are incredibly fast so can’t get a good look but appear to be black and slug like. Only appear when dark and slip between the cracks in our patio as soon as you shine a light on them. We don’t have a pond or water feature and live on the south east coast
Hi just a guess but they might be New Zealand flatworms. If so, they are an invasive species which predate on our native earthworms - not good news. There's an article HERE which you may be interested in. Personally, I've never seen one but I understand that they like hiding under stones, logs etc to keep damp otherwise they dry out.
Of course, they might be something else which I haven't thought of.