Found this on my living room wall. I live on the ground floor with lots of plants right outside the window. Have spotted a similar looking bug outside on the road near the area too.
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That looks like a Tropical leatherleaf slug.
The Tropical Leatherleaf (Laevicaulis alte) is found in both urban and forested areas. The animal is brownish, usually with darker speckles, and has a pale line on its back. It is not known whether it is native or not, though scientists think it is most likely introduced. It can grow to about 4cm long.
The Tropical Leatherleaf (Laevicaulis alte sensu lato, tak nang) is a flat, brown slug which often hides its tentacles. It is well adapted to the dry lowlands, hiding underground at daytime, and feeding on your plants at night.
On slugs and snails (Definitely check the picture here which matches yours quite nicely.)